scsi: ufs: Clean up and refactor clk-scaling feature

Manipulate clock scaling related stuff only if the host capability supports
clock scaling feature to avoid redundant code execution.

Link: https://lore.kernel.org/r/20210120150142.5049-4-stanley.chu@mediatek.com


Reviewed-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>