power: supply: max17040: Use regmap i2c

Rewrite i2c operations from i2c client read/write to regmap i2c. As
a result, most private functions now accept the private driver data
instead of an i2c client pointer.

Signed-off-by: Iskren Chernev <iskren.chernev@gmail.com>
Tested-by: Jonathan Bakker <xc-racer2@live.ca>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>