staging: omap-thermal: fix error checking

The omap_bandgap_get_sensor_data() function returns ERR_PTR(),
but it can also return NULL, in case of initilization, so we need to use
IS_ERR_OR_NULL() rather than only IS_ERR().

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>