[MIPS] Enable FAST-20 for onboard scsi

Both onboard controller of the O2 support FAST-20 transfer speeds,
but the bit, which signals that to the aic driver, isn't set. Instead
of adding detection code to the scsi driver, we just fake the missing
bit in the PCI config space of the scsi chips.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>