regulator: bd71828: remove get_voltage operation

Simplify LDO6 voltage getting on BD71828 by removing the
get_voltage call-back and providing the fixed voltage in
regulator_desc instead

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Suggested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20191219113444.GA28299@localhost.localdomain


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