mhi_bus: core: check event ring before scheduling work

MHI support MSI coalescing, meaning, same MSI vector can
be shared between multiple event rings. So, it's possible
event ring isr handler to get called with no new event. Confirm
there are pending events to process before scheduling work.

CRs-Fixed: 2211328
Change-Id: I696bf421854c728a33a6a34a1d98bfe8c80bb85f
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>