thermal-core: Fix invalid sensor request not accounted correctly

Add NULL check for sensor name before using it in sensor_get_id and
return NULL explicitly when sensor id is not valid in get_sensor of
thermal core.

Change-Id: I4200a5691183aa2d40ef8ed64554f0beb8ce6d0d
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>
msm: thermal: Disable hotplug monitor only if cpu-sensors < NR_CPUS

Some target variants have different core configuration but share
same dtsi bindings, hence it is not possible to configure differently
for these variants. Instead of checking number of cpus and number of
configured cpu-sensors are equal, check for atleast one cpu-sensor for
each cpu, ie cpu-sensors >= NR_CPUS at probe_cc.

Change-Id: Ie11107f780e24cc1e7879495ce0a1c0c5be412ab
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>