drivers: eth: gmac: Remove extra variable
drivers: eth: gmac: Remove extra variable

The msg_start variable was declared twice.

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

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