drivers: eth_sam_gmac: fix eth_sam_gmac_set_config()
drivers: eth_sam_gmac: fix eth_sam_gmac_set_config()

Fix handling of the return value by `eth_sam_gmac_set_config` function.
The function is used as a backend by
`net_mgmt(NET_REQUEST_ETHERNET_SET_MAC_ADDRESS, ...)` to change the
interface MAC address at run time.

Tested on sam_e70_xplained board.

Fixes #42151

Signed-off-by: Piotr Mienkowski <be7d9ab625d3cb5e3a4a3202f3529b954dba084c@gmail.com>

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