net/smscx5xx: use the device tree for mac address

This takes the MAC address for smsc75xx/smsc95xx USB network devices
from a the device tree. This is required to get a usable persistent
address on the popular beagleboard, whose hardware designers
accidentally forgot that an ethernet device really requires an a
MAC address to be functional.

The Raspberry Pi also ships smsc9514 without a serial EEPROM, stores
the MAC address in ROM accessible via VC4 firmware.

The smsc75xx and smsc95xx drivers are just two copies of the
same code, so better fix both.

[lkundrak@v3.sk: updated to use of_get_property() as per suggestion from
Arnd, reworded the message and comments a bit]

Tested-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: David S. Miller <davem@davemloft.net>