wl18xx: set default mac for untrimmed devices

Some 18xx untrimmed devises doesn't have a saved
oui and nic addresses on the chip registers, use
default oui addr in this case.

As a result the default mac address will be:
 addr 0: 08:00:28:00:00:01
 addr 1: 08:00:28:00:00:02
 addr 2: 08:00:28:00:00:03

Signed-off-by: Victor Goldenshtein <victorg@ti.com>