scsi: ufs: add checks before setting clk-gating states

Clock gating features can be turned on/off selectively which means its
state information is only important if it is enabled. This change makes
sure that we only look at state of clk-gating if it is enabled.

Change-Id: I201cb65d4a9712ba7c0f7022d7323cdf6dc144c9
Signed-off-by: Can Guo <cang@codeaurora.org>