[SCSI] WD33C93: let platform stub override no_sync/fast/dma_mode

SGI machines with WD33C93 allow usage of burst mode DMA, which increases
performance noticable. To make this selectable by the sgiwd93 stub,
setting the values for no_sync, fast and dma_mode has been moved to the
individual platform stubs.

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