net: broadcom: bcm4908_enet: handle -EPROBE_DEFER when getting MAC

Reading MAC from OF may return -EPROBE_DEFER if underlaying NVMEM device
isn't ready yet. In such case pass that error code up and "wait" to be
probed later.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20220915133013.2243-1-zajec5@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>