net: ethernet: dm9000: Handle -EPROBE_DEFER in dm9000_parse_dt()

The call to of_get_mac_address() can return -EPROBE_DEFER, for instance
when the MAC address is read from a NVMEM driver that did not probe yet.

Cc: H. Nikolaus Schaller <hns@goldelico.com>
Cc: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>