libata-pmp: implement qc_defer for command switching PMP support

Implement sata_pmp_qc_defer_cmd_switch() - standard qc_defer for
command switching PMP support.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
diff --git a/include/linux/libata.h b/include/linux/libata.h
index 3cfdb5f..ca296a5 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -947,6 +947,7 @@
 /*
  * PMP
  */
+extern int sata_pmp_qc_defer_cmd_switch(struct ata_queued_cmd *qc);
 extern void sata_pmp_read_init_tf(struct ata_taskfile *tf,
 				  struct ata_device *dev, int pmp, int reg);
 extern u32 sata_pmp_read_val(const struct ata_taskfile *tf);