msm: smd_pkt: Avoid race condition between close and read/write operations

If the close operation is performed by a different thread, then there
is a possibility of read/write operations getting stuck or accessing
some stale resources. The close operation is blocked if there is a read/
write operation is going on. The read/write operations check for
appropriate flags to see if a close operation is performed on a device.

Change-Id: I05ecce88b020c8da1658fe673be6e3ac001ae772
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>