net: Clarified the net_send_data() function return value
net: Clarified the net_send_data() function return value

If net_send_data() returns 0, then the buffer was sent ok
and the sending device driver has already freed the buffer.

If net_send_data() returns <0, then the buffer sending failed
and the caller can either try to fix the issue and re-send the
buffer, or the caller can unref the buffer. Normally caller
should unref the buffer in this case as there is not much that
can be done here.

Change-Id: I39d7b22bbb78a9c16fd037f3a066797e455c31fd
Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

tidyjiang8/zephyr-doc,zephyrproject-rtos/zephyr,kraj/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,mbolivar/zephyr,bigdinotech/zephyr,tidyjiang8/zephyr-doc,ldts/zephyr,zephyrproject-rtos/zephyr,rsalveti/zephyr,punitvara/zephyr,runchip/zephyr-cc3220,mbolivar/zephyr,Vudentz/zephyr,holtmann/zephyr,punitvara/zephyr,nashif/zephyr,erwango/zephyr,sharronliu/zephyr,erwango/zephyr,rsalveti/zephyr,fractalclone/zephyr-riscv,bigdinotech/zephyr,tidyjiang8/zephyr-doc,zephyriot/zephyr,aceofall/zephyr-iotos,holtmann/zephyr,galak/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,Vudentz/zephyr,fbsder/zephyr,fractalclone/zephyr-riscv,punitvara/zephyr,erwango/zephyr,runchip/zephyr-cc3200,galak/zephyr,galak/zephyr,mbolivar/zephyr,erwango/zephyr,rsalveti/zephyr,erwango/zephyr,rsalveti/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,kraj/zephyr,punitvara/zephyr,finikorg/zephyr,zephyriot/zephyr,sharronliu/zephyr,ldts/zephyr,tidyjiang8/zephyr-doc,zephyrproject-rtos/zephyr,fractalclone/zephyr-riscv,nashif/zephyr,finikorg/zephyr,mbolivar/zephyr,bigdinotech/zephyr,runchip/zephyr-cc3220,rsalveti/zephyr,pklazy/zephyr,galak/zephyr,ldts/zephyr,fbsder/zephyr,finikorg/zephyr,bboozzoo/zephyr,bboozzoo/zephyr,fractalclone/zephyr-riscv,pklazy/zephyr,punitvara/zephyr,holtmann/zephyr,tidyjiang8/zephyr-doc,explora26/zephyr,explora26/zephyr,runchip/zephyr-cc3200,Vudentz/zephyr,holtmann/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,bigdinotech/zephyr,sharronliu/zephyr,zephyriot/zephyr,explora26/zephyr,mbolivar/zephyr,fractalclone/zephyr-riscv,explora26/zephyr,kraj/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,bigdinotech/zephyr,kraj/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,sharronliu/zephyr,Vudentz/zephyr,explora26/zephyr,kraj/zephyr,runchip/zephyr-cc3200,bboozzoo/zephyr,nashif/zephyr,aceofall/zephyr-iotos,holtmann/zephyr,runchip/zephyr-cc3220,sharronliu/zephyr,bboozzoo/zephyr,runchip/zephyr-cc3200,GiulianoFranchetto/zephyr,runchip/zephyr-cc3200,Vudentz/zephyr,pklazy/zephyr,runchip/zephyr-cc3220,Vudentz/zephyr,bboozzoo/zephyr,nashif/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,pklazy/zephyr,pklazy/zephyr