leds: qpnp-flash: Fix possible race condition in debugfs

There is a possible race condition when debugfs files are concurrently
accessed by multiple threads. Fix this.

CRs-Fixed: 1109420, 1109326
Change-Id: I19e9107079ac8d039b12a37ae612727f824552d4
Signed-off-by: Ankit Sharma <ansharma@codeaurora.org>