scsi: ufs: Serialize eh_work with system PM events and async scan

Serialize eh_work with system PM events and async scan to make sure eh_work
does not run in parallel with them.

Link: https://lore.kernel.org/r/1606910644-21185-2-git-send-email-cang@codeaurora.org
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Reviewed-by: Asutosh Das <asutoshd@codeaurora.org>
Reviewed-by: Hongwu Su <hongwus@codeaurora.org>
Signed-off-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>