brcm47xx: fix unaligned access to mac address

The mac address should be aligned to u16 to prevent an unaligned access
in drivers/ssb/pci.c where it is casted to __be16.

This closes #9824.


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35618 3c298f89-4303-0410-b956-a3cf2f4a3e73