leds: qpnp-flash: Fix Use-after-free(UAF) for debugfs

Fix UAF where two threads can open and close the same file. Second
open will cause the private data for the first file to be overwritten.
When the first file is closed and the private data is freed, this makes
the now-shared private data OOB for the second thread.

CRs-Fixed: 1109763
Change-Id: I1c4618d5be99e140abf0f3ea0d7f485897db5ab2
Signed-off-by: Ankit Sharma <ansharma@codeaurora.org>