[ar71xx] Fix routerstation / routerstation pro redboot FIS probing

Some changes back in ye olde times somewhere has changed the default
block size the flash device exposes. So, the default geom redboot
FIS probing (to find the partition table structure in flash!)
is no longer finding it.

So, force it to probe at the last 64k of flash regardless of the
underlying flash block size.

Tested:

• Ubiquiti Routerstation pro, boots -HEAD MIPS