regulator: da9052: Revert se apply_[reg|bit] with regmap based voltage_sel operations

This reverts commit 68f75060.

Michael reported that with this patch we loose the fix_io code path from
da9052_reg_update. Thus revert it.

Reported-by: Michael Grzeschik <mgr@pengutronix.de>
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>