[SCSI] aic79xx: user visible misuse wrong SI units (not disk size!)

MHZ not Mhz for SI unit pedants

Closes bug #6422

Signed-off-by: Alan Cox <alan@redhat.com>
Acked-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>