regulator: s5m8767: Use DIV_ROUND_UP to calculate selector

Integer division may truncate the result.
Use DIV_ROUND_UP to ensure new voltage setting falls within specified range.

Also properly handle the case min_vol < desc->min to ensure we don't return
negative value for selector.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>