scsi: ufs: Get a TM service response from the correct offset

When any UFS host controller receives a TM(Task Management) response
from a UFS device, UFS driver has been recognize like receiving a
message of "Task Management Function Complete"(00h) in all cases, so
far.  That means there is no pending task for a tag of the TM request
sent before in the UFS device.  That's because the byte offset 6 in TM
response which has been used to get a TM service response so far
represents just whether or not a TM transmission passes.

Regarding UFS spec, the correct byte offset to get TM service response
is 15, not 6.

I tested that UFS driver responds properly for the TM response from a
UFS device with an reference board with exynos8890, as follow: No
pending task -> Task Management Function Complete (00h) Pending task ->
Task Management Function Succeeded (08h)

[mkp: applied by hand]

Signed-off-by: Kiwoong Kim <kwmad.kim@samsung.com>
Signed-off-by: HeonGwang Chu <hg.chu@samsung.com>
Tested-by: : Kiwoong Kim <kwmad.kim@samsung.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>