power: supply: bq25890: Fix usb-notifier probe and remove races

There are 2 races surrounding the usb-notifier:

1. The notifier, and thus usb_work, may run before the bq->charger
   power_supply class device is registered. But usb_work may call
   power_supply_changed() which relies on the psy device being registered.

2. usb_work may be pending/running at remove() time, so it needs to be
   cancelled on remove after unregistering the usb-notifier.

Fix 1. by moving usb-notifier registration to after the power_supply
registration.

Fix 2. by adding a cancel_work_sync() call directly after the usb-notifier
unregistration.

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>