power: supply: axp20x_usb_power: Only poll while offline

Investigation on the AXP803 shows that VBUS_PLUGIN/VBUS_REMOVAL IRQs are
triggered on the rising/falling edge of AXP20X_PWR_STATUS_VBUS_USED. The
reason IRQs do not arrive while N_VBUSEN/DRIVEVBUS is high is because
AXP20X_PWR_STATUS_VBUS_USED also never goes high.

This also means that if VBUS is online, a VBUS_REMOVAL IRQ is received
immediately on setting N_VBUSEN/DRIVEVBUS high (and VBUS_PLUGIN shortly
after it is set back low). This was also verified to be the case when
manually offlining VBUS through AXP20X_VBUS_PATH_SELECT.

As long as VBUS is online, a present->absent transition necessarily
implies an online->offline transition. Since will cause an IRQ, there is
no need to poll while VBUS is online.

To ensure the driver's view of VBUS online status remains accurate,
unconditionally poll once when receiving an IRQ and when resuming. If
VBUS is still online at that time, polling will cease until the next
VBUS_REMOVAL IRQ.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>