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.

Bug: 33781694
CRs-Fixed: 1109420, 1109326
Change-Id: I19e9107079ac8d039b12a37ae612727f824552d4
Signed-off-by: ansharma <ansharma@codeaurora.org>
Signed-off-by: Biswajit Paul <biswajitpaul@codeaurora.org>
Signed-off-by: Steve Pfetsch <spfetsch@google.com>