drivers: net: loopback: Fix possible double unref
drivers: net: loopback: Fix possible double unref

If we could not send the packet, then do not release the net_pkt
as that will be released in net_if.c:net_if_tx() if driver send()
fails.

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

zephyriot/zephyr,ldts/zephyr,finikorg/zephyr,mbolivar/zephyr,Vudentz/zephyr,nashif/zephyr,kraj/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,zephyriot/zephyr,punitvara/zephyr,mbolivar/zephyr,zephyriot/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,finikorg/zephyr,punitvara/zephyr,punitvara/zephyr,mbolivar/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,finikorg/zephyr,kraj/zephyr,Vudentz/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,mbolivar/zephyr,explora26/zephyr,galak/zephyr,explora26/zephyr,zephyriot/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,kraj/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,kraj/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr