thermal: qpnp-adc: Add checks for reading the correct index.

Add a robust check to verify if the channel being requested
for threshold monitoring exists when setting the thresholds
for the channel. Also fix using the correct revision register
to determine whether 5 or 8 channels are supported.

Change-Id: I5b8b1e632eb82b704599247921e53075ad8c5c0c
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>