regulator: mp5416: Convert to use regulator_set_ramp_delay_regmap

Use regulator_set_ramp_delay_regmap instead of open-coded.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Message-Id: <20210522023018.2025188-1-axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@sirena.org.uk>