[SCSI] update spi transport class so that u320 Domain Validation works

There are several extra things that have to be considered when running
Domain Validation on a u320 target (notably how you fall back).

Hopefully this should help us when someone adds this transport class to
aic79xx.

I've tested this on the lsi1030, so I know it works correctly up to
u320.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>