ath79: ag71xx: support probe defferal for getting MAC address

ath79: ag71xx: support probe defferal for getting MAC address

Currently, of_get_ethdev_address() return is checked for any return error
code which means that trying to get the MAC from NVMEM cells that is backed
by MTD will fail if it was not probed before ag71xx.

So, lets check the return error code for EPROBE_DEFER and defer the ag71xx
probe in that case until the underlying NVMEM device is live.

Link: https://github.com/openwrt/openwrt/pull/15752
Signed-off-by: Robert Marko <robimarko@gmail.com>