scsi: ufs: error out all issued requests after shutdown

After PON (power-off notification) is sent, no requests
should be issued to the driver.
This change errors out all requests except from shutdown
context after shutdown is initiated.

Change-Id: Id499abc27303bfed72fab4d61abb872bad7d9043
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
[cang@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Can Guo <cang@codeaurora.org>