net: bgmac-bcma: handle deferred probe error due to mac-address

Due to the inclusion of nvmem handling into the mac-address getter
function of_get_mac_address() by
commit d01f449c ("of_net: add NVMEM support to of_get_mac_address")
it is now possible to get a -EPROBE_DEFER return code. Which did cause
bgmac to assign a random ethernet address.

This exact issue happened on my Meraki MR32. The nvmem provider is
an EEPROM (at24c64) which gets instantiated once the module
driver is loaded... This happens once the filesystem becomes available.

With this patch, bgmac_probe() will propagate the -EPROBE_DEFER error.
Then the driver subsystem will reschedule the probe at a later time.

Cc: Petr Štetiar <ynezz@true.cz>
Cc: Michael Walle <michael@walle.cc>
Fixes: d01f449c

 ("of_net: add NVMEM support to of_get_mac_address")
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>