libata: fix boot panic with SATAPI devices on non-SFF HBAs

The kernel now panics reliably on boot if you have a SATAPI device
connected.

The problem was introduced by the libata merge trying to pull out all
the SFF code into a separate module.  Unfortunately, if you're a satapi
device you usually need to call atapi_request_sense, which has a bare
invocation of a SFF callback which is NULL on non-SFF HBAs.  Fix this by
making the call conditional.
Signed-off-by: NJames Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/ata/libata-scsi.c

@@ -2394,7 +2394,8 @@ static void atapi_request_sense(struct ata_queued_cmd *qc)
 	memset(cmd->sense_buffer, 0, SCSI_SENSE_BUFFERSIZE);
 
 #ifdef CONFIG_ATA_SFF
-	ap->ops->sff_tf_read(ap, &qc->tf);
+	if (ap->ops->sff_tf_read)
+		ap->ops->sff_tf_read(ap, &qc->tf);
 #endif
 
 	/* fill these in, for the case where they are -not- overwritten */