power: supply: bq27xxx: After charger plug in/out wait 0.5s for things to stabilize

bq27xxx_external_power_changed() gets called when the charger is plugged
in or out. Rather then immediately scheduling an update wait 0.5 seconds
for things to stabilize, so that e.g. the (dis)charge current is stable
when bq27xxx_battery_update() runs.

Fixes: 740b755a

 ("bq27x00: Poll battery state")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>