drivers/ethernet: Remove double unref from stellaris driver
drivers/ethernet: Remove double unref from stellaris driver

It's now either L2 which unref the pkt on successful tx, or net_if on
error.

Also removing pkt->frags check, net_core.c:net_send_data() does it
already.

And using data_len in logging instead of net_pkt_get_len(), which one is
currently greedy (it goes over all net_bufs).

Signed-off-by: Tomasz Bursztyka <ba81a3a719836727e6857ae83462b9f52ec41006@linux.intel.com>

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