power: supply: axp20x_usb_power: add function to get max current

To prepare for a new PMIC, factor out the code responsible of returning
the maximum current to axp20x_get_current_max.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>