This patch fixes below issues when choosing selector:

1. Current code returns negative selector if min_uV < 900000 which is wrong.
   For example, it is possible to satisfy the request with selector = 1 if
   the requested min_uV is 850000.
2. Current code may select a voltage lower than requested min_uV.
   For example, if the requested min_uV is 945000, current code chooses
   selector = 1 which is lower than requested min_uV.
   DIV_ROUND_UP to avoid this case.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>