net: ethernet: Fix build issue with C++
net: ethernet: Fix build issue with C++

When compiling with CPP, compiler complains about implicit type
conversions. Fix them.

Signed-off-by: Loic Poulain <7dbc6498a90ff1ada42fc224430b4f763baa5c5c@linaro.org>

Vudentz/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr