staging: omap-thermal: rename add_hyst to omap_bandgap_add_hyst

This patch improves the add_hyst function by:
. Renaming it to omap_bandgap_add_hyst
. Moving it to the ADC conversion functions section
. Changing its signature to follow the driver standard

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