[SCSI] aic7xxx: cap maxsync according to correct card limits

Not doing this can cause cards less than u160 capable to send out PPR
offers to devices they can't then deliver on ... causing some devices to
get a bit confused.  Fix by capping the start syncrate at the
appropriate level according to the card capabilities.

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