power: supply: axp20x_usb_power: Use regmap field for VBUS disabling

The POWER_SUPPLY_PROP_ONLINE property allows controlling the VBUS
enable state on supported PMICs. Switch to regmap fields to reduce
dependence on variant IDs.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>