libata: fix hw_sata_spd_limit initialization

hw_sata_spd_limit used to be incorrectly initialized to zero instead
of UINT_MAX if SPD is zero in SControl register.  This breaks PHY
speed down.  Fix it.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/libata-core.c

@@ -6317,7 +6317,8 @@ int ata_host_register(struct ata_host *host, struct scsi_host_template *sht)
 		/* init sata_spd_limit to the current value */
 		if (sata_scr_read(ap, SCR_CONTROL, &scontrol) == 0) {
 			int spd = (scontrol >> 4) & 0xf;
-			ap->hw_sata_spd_limit &= (1 << spd) - 1;
+			if (spd)
+				ap->hw_sata_spd_limit &= (1 << spd) - 1;
 		}
 		ap->sata_spd_limit = ap->hw_sata_spd_limit;