msm: mhi_dev: Skip channel doorbell interrupts on unused channels

Skip channel doorbell interrupts on uninitialized channels and continue
processing other valid channel interrupts instead of erroring-out.

Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>
Change-Id: If90eb6314628419d4823cf7023df0471f18a6071