regulator: max8998: Ensure enough delay time for max8998_set_voltage_buck_time_sel

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>
Cc: stable@vger.kernel.org