Instead of doing the read-modify-update cycle by hand when updating a
register use regmap_update_bits().

This also means we can now remove uda134x_read_reg_cache() and
uda134x_write() since they are unused.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>