scsi: ufs: Fix a possible NULL pointer issue

During system resume/suspend, hba could be NULL. In this case, do not touch
eh_sem.

Fixes: 88a92d6a

 ("scsi: ufs: Serialize eh_work with system PM events and async scan")
Link: https://lore.kernel.org/r/1610594010-7254-2-git-send-email-cang@codeaurora.org
Acked-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>