staging: omap-thermal: name adc_to_temp_conversion in a better way

Rename adc_to_temp_conversion to omap_bandgap_adc_to_mcelsius. This name,
though longer, describes better the function.

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