thermal: qpnp-temp-alarm: correct thermal sensor registration check

Correct the error check for the thermal_zone_of_sensor_register()
call to check for IS_ERR() instead of NULL.

Change-Id: Ie021b4e7998aa970ce3a317cf9d6d9cc154ee4d0
Signed-off-by: David Collins <collinsd@codeaurora.org>