[libata] ahci: Add SATA GEN3 related messages

The present AHCI driver seems to support SATA GEN 3 speed, but the related
messages should be modified.

Signed-off-by: Shane Huang <shane.huang@amd.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index f178a45..9ae5835 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -1007,6 +1007,7 @@
 	static const char * const spd_str[] = {
 		"1.5 Gbps",
 		"3.0 Gbps",
+		"6.0 Gbps",
 	};
 
 	if (spd == 0 || (spd - 1) >= ARRAY_SIZE(spd_str))