regulator: s2mps11: Fix setting ramp_delay

Current code has wrong mask and val arguments for updating ramp_delay. Fix it.
Also ensure the return value of get_ramp_delay() won't greater than 3 because
the mask field for ramp_val only takes 2 bits.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>