[SCSI] aic7xx: mitigate HOST_MSG_LOOP invalid SCB ff panic

The panic occurs if we get a MSGIN or MSGOUT for an unidentified SCB
(meaning we didn't identify the outstanding command it was for).  For
MSGIN this is wrong because it could be an unsolicited negotiation
MSGIN from the target.

Still panic on unsolicited MSGOUT because this would represent a
mistake in the negotiation phases.  However, we should fix this as
well.  The specs say we should go to bus free for unexpected msgin.

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