power_supply: max17042: Use regmap_update_bits instead read and write

Consolidate regmap_read() and regmap_write() into one
regmap_update_bits() call. This is more readable and safer because
regmap's mutex will prevent any concurrent access to modified registers
(the concurrent access could happen through max17042_init_chip() in
scheduled work).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>