ufs: ensure clk gating work is finished before module unloading

When dynamic clk gating feature is enabled, delayed workqueue machanism
is used in order to detect certain period of inactivity.  But there is no
guarantee that scheduled gating work is completed before module unloading.
So it can cause kernel crash by accessing memory after it was freed.

Fix it by cancelling clk gating and ungating works and ensure that its
execution is finished before module unloading.

Signed-off-by: Akinobu Mita <mita@fixstars.com>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>