scsi: ufs: Fix irq return code

Return IRQ_HANDLED only if the irq is really handled, this will help in
catching spurious interrupts that go unhandled.

Link: https://lore.kernel.org/r/1573798172-20534-6-git-send-email-cang@codeaurora.org


Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>