regulator: tps6105x: Convert to use regmap helper functions

Since commit 7e507119 ("mfd: tps6105x: Use i2c regmap to access
registers"), we can use regmap helper functions instead of open coded.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Tested-by: Denis Grigoryev <grigoryev@fastwel.ru>
Signed-off-by: Mark Brown <broonie@kernel.org>