mhi: core: Prevent doorbell with invalid tre

In the cases of HW accelerated channels, there
is a possibility that the db_mode has changed while
a queue is in progress. This can happen when
mhi_queue_dma is scheduled out and an OOB event
is parsed.
Fix this by determining whether to ring the
doorbell or not without releasing the lock to
ensure persistence of db_mode.

Change-Id: I586937057ff0b3329813c10d5f4337a2df887fd1
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>