power: supply: axp20x_usb_power: Init work before enabling IRQs

The IRQ handler calls mod_delayed_work() on power->vbus_detect. However,
that work item is not initialized until after the IRQs are enabled. If
an IRQ is already pending when the driver is probed, the driver calls
mod_delayed_work() on an uninitialized work item, which causes an oops.

Fixes: bcfb7ae3 ("power: supply: axp20x_usb_power: Only poll while offline")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>