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 <piotr.mienkowski@gmail.com>