power: qpnp-fg-gen3: Fix race condition in Time To Empty (TTE)

There is a possible race condition in TTE where it might cause
Out of Bound array access because the buffer gets cleared.
Fix this by adding a lock to TTE.

Change-Id: I2074c42aac9655967a8f77a84362507c3954cf20
Signed-off-by: Umang Chheda <uchheda@codeaurora.org>