power: supply: axp20x_usb_power: Remove variant IDs from VBUS polling check

Use an explicit boolean flag instead of a check based on the
variant ID. Since this is the last use of variant IDs in the
driver, also remove the IDs.

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