ar71xx: Fix offset to WMAC address for 8devices Lima

ar71xx: Fix offset to WMAC address for 8devices Lima

The ART partition of the Lima board stores exactly three mac addresses:

* 0x0: eth0
* 0x6: eth1
* 0x1002: wmac

The first two are correctly assigned in the mach file but the latter points
to 0x800. But this position is set to ff:ff:ff:ff:ff:ff. Luckily, the
driver falls back in ath9k_hw_init_macaddr to the EEPROM mac address when
it doesn't find a valid mac address in the platform_data.

Remove this bogus offset to the ART partition to directly load the wmac via
the EEPROM data in the ART partition.

Signed-off-by: Sven Eckelmann <sven@narfation.org>