regulator: bd71815: Use defined mask values

Consistently use the defines for buck control mask values.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/YVqpujZLZmaiqwe8@fedora


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