scsi: ufs: update dev ref clock gating wait time

This change updates the wait time before turning off the reference clock.
The wait time of UFS version 3.0 and onward devices can be read out from
their device attributes. Otherwise, fix the wait time to 100 micro seconds.

Change-Id: Ie5085e62ef3efa2bfaa3f632593e4ade52ffd645
Signed-off-by: Can Guo <cang@codeaurora.org>