libata-sff: prd is BMDMA specific

struct ata_prd and ap->prd are BMDMA specific.  Add bmdma_ prefix to
them and move them inside CONFIG_ATA_SFF.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
diff --git a/include/linux/ata.h b/include/linux/ata.h
index 700c5b9b..fe6e681 100644
--- a/include/linux/ata.h
+++ b/include/linux/ata.h
@@ -467,7 +467,7 @@
 
 /* core structures */
 
-struct ata_prd {
+struct ata_bmdma_prd {
 	__le32			addr;
 	__le32			flags_len;
 };