mhi: core: Mark unprocessed events as STALE

MHI clients permitted to reset channels asynchronous
to pending data. It's possible after channel reset
there are pending events for that particular channel.
Mark all those events as stale, such that MHI event
process thread will ignore them.

CRs-Fixed: 1074226
Change-Id: I86547aa2c5d7ad5095807cfe87378f5540c05a66
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>