regulator: tps65910: use regmap accessors

Use regmap accessors directly for register manipulation - removing
one layer of abstraction.
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lore.kernel.org/r/e82886d0f8f5131c9fccf2a17e3a15acce507d6f.1601164493.git.mirq-linux@rere.qmqm.plSigned-off-by: Mark Brown <broonie@kernel.org>