regulator: max8998: Ensure enough delay time for max8998_set_voltage_buck_time_sel

commit 81d0a6ae7befb24c06f4aa4856af7f8d1f612171 upstream.

Use DIV_ROUND_UP to prevent truncation by integer division issue.
This ensures we return enough delay time.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
[bwh: Backported to 3.2: delay is done by driver, not returned to the caller]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>