qcom: fg-util: 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: 1105395
Change-Id: Iea5a8d659da231a81630d8f2302714e69d35de61
Signed-off-by: ansharma <ansharma@codeaurora.org>