regulator: axp20x: fix set_ramp_delay for AXP209/dcdc2

Initial commit of set_ramp_delay feature was missing an assignment which
should have populated slew_rate table for dcdc2 regulator. Add it.

Fixes: d29f54df ("regulator: axp20x: add support for set_ramp_delay for AXP209")

Signed-off-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Mark Brown <broonie@kernel.org>