scsi: ufs: fix bugs for UFS debugfs entry power_mode

Remove support to change power_mode to slow/slowauto mode. Meanwhile,
changing UFS gear through debugfs entry power_mode may run into race
condition with UFS devfreq scaling thread. This change adds r/w semaphore
to synchronize the gear changing action with the devfreq scaling thread.
Moreover, gear scaling needs to be taken care of along with the clk freq
scaling. This change also adds the logic to implement so.

Change-Id: I60c84ea7bef11c93cbed3f803cf416ff35313a0b
Signed-off-by: Can Guo <cang@codeaurora.org>