mhi: core: do not acquire channel mutex to notify clients

A deadlock is seen when an MHI client attempts to start channels
for transfer and MHI core driver tries to notify the client of a
a status change. Remove mutex from notify path as it should not
be required since we already use the safe list iteration option.

CRs-Fixed: 2521963
Change-Id: I6388547fb3e0daa1763974a1a8579a3831e3679a
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>