mhi: devices: uci: release lock as last step

mhi_uci_release and mhi_uci_remove can get called in parallel and
protected by a mutex.  Both functions can release the same
resources, therefore all code execution must be inside the mutex
to avoid use after free errors.

CRs-Fixed: 2340364
Change-Id: I7786cab2897db2ec7ad1380e1920494627cbea73
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>