AA: ar71xx: Routerboard 751 Mac Address Offset Fix

Backport of r35519.

We utilize many Routerboard 751's and discovered that our latest batch
of RB751's would not initialize the wireless radio. We have determined
Mikrotik has changed where the mac address was located inside hardconfig.
As such we utilize "routerboot_find_tag" to find the location of the mac
address. We should remove "RB751_MAC_ADDRESS_OFFSET" as it is ambiguous
by machine manufacturing date. The newer batch of RB751's that we received
had a RB751_MAC_ADDRESS_OFFSET 0x10.

Signed-off-by: Davey Hutchison <dhutchison@bluemesh.net>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@35531 3c298f89-4303-0410-b956-a3cf2f4a3e73