regulator: s2mpa01: Don't check enable_shift before setting enable ramp rate

Current code misses updating the register when enable_shift is 0.
e.g. S2MPA01_BUCK4_RAMP_EN_SHIFT is 0.

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