[SCSI] Negotiate correctly with async-only devices

When we got a device only capable of async, we would zero out goal->period
which would cause us to try PPR negotiations.  Leave goal->period alone,
and check goal->offset before doing PPR.  Kudos to Daniel Forsgren for
figuring this out.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>