mac80211: tweak mac address allocation

Apparently there are some broken devices out there that have the local
bit already set in the factory MAC address. Move the | 0x2 over to the
other side of the xor operation to fix this without affecting devices
with proper addresses.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r37269

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@37270 3c298f89-4303-0410-b956-a3cf2f4a3e73