input: qcom-hv-haptics: Add a property to specify FIFO empty threshold

In FIFO mode playing, when the number of samples in the FIFO memory
is less than the FIFO empty threshold, an interrupt is triggered for
notifying the driver to refill the FIFO memory if there is still any
samples pending to be played. FIFO empty threshold can be a tuning
parameter for the driver to refill the FIFO memory timely before the
hardware drains out the FIFO samples to avoid an intermittent pause
in vibration effect. Add a DT property to specify FIFO empty threshold.
Also, expose it under debugfs for modifying it runtime.

Meanwhile, return proper values when creating debugfs nodes failed.

Change-Id: I3ac9e6210173ab6878b4487959bc4b3fc3f20ab0
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>