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

Rename temp_to_adc_conversion to omap_bandgap_mcelsius_to_adc. This name,
though longer, describes better the function.

This patch also changes this function signature so the function follows
the style of this file.

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