[SCSI] sgiwd93: use cached memory access to make driver work on IP28

SGI IP28 machines would need special treatment (enable adding addtional
wait states) when accessing memory uncached. To avoid this pain I
changed the driver to use only cached access to memory.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>