regulator: Fix the logic of s5m8767_set_voltage_time_sel

This patch includes below fixes:
1. The mask variable is not used at all here, remove it.
2. We already have the new_sel and old_sel, simply returns the delay by:
  DIV_ROUND_UP(desc->step * (new_sel - old_sel), s5m8767->ramp_delay);

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