thermal: qcom: Use thermal_zone_get_crit_temp() in qpnp_tm_init()

Modify qpnp_tm_init() to use thermal_zone_get_crit_temp() to get the
critical trip temperature instead of iterating over trip indices and
using thermal_zone_get_trip() to get a struct thermal_trip pointer
from a trip index until it finds the critical one.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://patch.msgid.link/7712228.EvYhyI6sBW@rjwysocki.net