libata: make SFF support optional

Now that SFF support is completely separated out from the core layer,
it can be made optional.  Add CONFIG_ATA_SFF and let SFF drivers
depend on it.  If CONFIG_ATA_SFF isn't set, all codes in libata-sff.c
and data structures for SFF support are disabled.  This saves good
number of bytes for small systems.

Signed-off-by: Tejun Heo <htejun@gmail.com>

drivers/ata/libata-scsi.c

@@ -2393,7 +2393,9 @@ static void atapi_request_sense(struct ata_queued_cmd *qc)
 	/* FIXME: is this needed? */
 	memset(cmd->sense_buffer, 0, SCSI_SENSE_BUFFERSIZE);
 
+#ifdef CONFIG_ATA_SFF
 	ap->ops->sff_tf_read(ap, &qc->tf);
+#endif
 
 	/* fill these in, for the case where they are -not- overwritten */
 	cmd->sense_buffer[0] = 0x70;