regulator: Ensure enough enable time for max8649

Integer division may truncate the result, thus max8649_enable_time() may return
slightly shorter enable time. This patch fixes it.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>