uio: msm_sharedmem: Restrict debugfs write to root.

Restrict the write permissions to the debugfs entries
only to the root user and set the permissions as 0200.

Change-Id: I5339cf531de7b26dacffe5c3e9010ee892193bb1
Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>