regulator: core: Don't skip set_voltage_time when ramp delay disabled

The current code assumes that only the ramp_delay is used to determine
the time needed for the voltage to stabilize. This may be true for the
calculation done by regulator_set_voltage_time_sel(), however regulators
can implement their own set_voltage_time_sel() op which would be skipped
if no ramp delay is specified. Remove the check in
_regulator_do_set_voltage(), the functions calculating the ramp delay
return 0 anyway when the ramp delay is not configured.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>