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.

Change-Id: Icc2a373f3f4599887c4f86632107ed11e0525153
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
[cang@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Can Guo <cang@codeaurora.org>