mhi: core: only accumulate transfer complete events as processed events

When a client ask MHI host to processed X number of events, only count
transfer events as part of processed events. All other events are internal
events.

CRs-Fixed: 2018324
Change-Id: I4cfcae0ba187348442c3fc45b76d7c4b01f687bd
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>