staging: omap-thermal: fix error check in omap_thermal_expose_sensor() and in omap_thermal_register_cpu_cooling().

The omap_bandgap_get_sensor_data() function returns ERR_PTR(), so we need to use
IS_ERR() rather than a NULL check.
Signed-off-by: Cyril Roelandt <tipecaml@gmail.com>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>