regulator: fan53555: use set_ramp_delay to set the ramp up slew rate

The regulator constraints already provide a field for the ramp_delay, so
there is no need to set this manually. Therefore implement the set_ramp_delay
callback and convert the pdata value to the constraint value if necessary.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@kernel.org>