thermal: imx: Drop critical trip check from imx_set_trip_temp()

Because the IMX thermal driver does not flag its critical trip as
writable, imx_set_trip_temp() will never be invoked for it and so the
critical trip check can be dropped from there.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/2272035.iZASKD2KPV@rjwysocki.net