regulator: axp20x: Fix axp20x_set_ramp_delay

Current code set incorrect bits when set ramp_delay for AXP20X_DCDC2,
fix it.

Fixes: d29f54df

 ("regulator: axp20x: add support for set_ramp_delay for AXP209")
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/20191221081049.32490-1-axel.lin@ingics.com
Signed-off-by: Mark Brown <broonie@kernel.org>