mhi: core: release channel locks before removing mhi devices

There could be a deadlock between mhi device remove, and client
calling channel start\stop operation because of common lock. To
avoid this deadlock, release channel lock before calling client
device remove function.

CRs-Fixed: 2281958
Change-Id: I332fee417c3ebaae113e11375d10f4487b2194a2
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>