mhi: core: improve time synchronization events handling

Validate event ring accesses before walking special event rings and
update processing time synchronization such that a doorbell is rung and
future requests are queued until a response is received. Also, ensure
events are only processed when they are pending.

Change-Id: Ib8c9385530f822af3e53efb96b82c0f26c3438b7
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>