thermal: of: Remove bogus type qualifier for of_thermal_get_trip_points()

With gcc 4.1.2, 4.2, and 4.2.4 (4.4 and later are OK):

drivers/thermal/thermal_core.h:110: warning: type qualifiers ignored on function return type

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: ce8be778

 ("thermal: of: Extend of-thermal to export table of trip points")
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>