drivers: ethernet: Fix variable type
drivers: ethernet: Fix variable type

net_recv_data returns an int, so we should not use an unsigned type.

Signed-off-by: Flavio Ceolin <979b9165500b0741b9d0500e2efd74fc1547bff7@intel.com>

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