msm: mhi_dev: Serialize UCI open and close file node operations

Multiple clients might perform open and close file operations
simultaneously on the same file node created by UCI. To avoid
race conditions which might result in invalid accesses or crashes,
the open and close operations are serialized.

Change-Id: Ic6a290a2c6af25bcd60983a7b531bf1287201821
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>