regulator: max8997: Remove set_voltage_time_sel setting for max8997_ldo_ops

max8997_set_voltage_ldobuck_time_sel() returns 0 for all LDOs.
Thus remove set_voltage_time_sel setting for max8997_ldo_ops.

max8997_set_voltage_ldobuck_time_sel() is only used for max8997_buck_ops now,
rename it to max8997_set_voltage_buck_time_sel().

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