This function finds the smallest voltage that falls within the specified range,
and then returns the selector. This rename makes the intention more clear.

Also remove unneeded local variables min_vol and max_vol in s5m8767_set_voltage
and s5m8767_set_voltage_buck.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sangbeom Kim <sbkim73@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>