scsi: ufs: do not destroy clk scaling work queue if it is NULL

Function ufshcd_hba_exit might be called to destroy clk scaling work queue
if failure happends in function ufshcd_init before clk gating work queue
is created. This change fixs it by skipping destroying clk scaling work
queue if it is NULL.

Change-Id: Ifb1c46f490749e4afe7cea331c29d073175f68ef
Signed-off-by: Can Guo <cang@codeaurora.org>