libata: Fix display of sata speed

6.0 Gbps link speed was not decoded properly:
speed was reported at 3.0 Gbps only.

Tested: On a machine where libata reports 6.0 Gbps in
        /var/log/messages:
    ata1: SATA link up 6.0 Gbps (SStatus 133 SControl 300)

    Before:
    	cat /sys/class/ata_link/link1/sata_spd
    	3.0 Gbps
    After:
    	cat /sys/class/ata_link/link1/sata_spd
    	6.0 Gbps

Signed-off-by: Gwendal Grignou <gwendal@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org