mhi: core: release pm_lock before removing MHI devices

During MHI shutdown deadlock could occur between device remove
and device probe.  During remove, MHI host aquire pm_lock and
attempts to grab device lock while in probe, base driver acquire
device lock and attempts to grab pm lock. To avoid this dead lock,
release pm lock before removing devices.

CRs-Fixed: 2359701
Change-Id: I1fa28e3bc6104fd5de95d7ee82dd4a2bc1b9905f
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>