power: supply: bq25890: Fix external_power_changed race

bq25890_charger_external_power_changed() dereferences bq->charger,
which gets sets in bq25890_power_supply_init() like this:

  bq->charger = devm_power_supply_register(bq->dev, &bq->desc, &psy_cfg);

As soon as devm_power_supply_register() has called device_add()
the external_power_changed callback can get called. So there is a window
where bq25890_charger_external_power_changed() may get called while
bq->charger has not been set yet leading to a NULL pointer dereference.

This race hits during boot sometimes on a Lenovo Yoga Book 1 yb1-x90f
when the cht_wcove_pwrsrc (extcon) power_supply is done with detecting
the connected charger-type which happens to exactly hit the small window:

  BUG: kernel NULL pointer dereference, address: 0000000000000018
  <snip>
  RIP: 0010:__power_supply_is_supplied_by+0xb/0xb0
  <snip>
  Call Trace:
   <TASK>
   __power_supply_get_supplier_property+0x19/0x50
   class_for_each_device+0xb1/0xe0
   power_supply_get_property_from_supplier+0x2e/0x50
   bq25890_charger_external_power_changed+0x38/0x1b0 [bq25890_charger]
   __power_supply_changed_work+0x30/0x40
   class_for_each_device+0xb1/0xe0
   power_supply_changed_work+0x5f/0xe0
  <snip>

Fixing this is easy. The external_power_changed callback gets passed
the power_supply which will eventually get stored in bq->charger,
so bq25890_charger_external_power_changed() can simply directly use
the passed in psy argument which is always valid.

Fixes: eab25b4f ("power: supply: bq25890: On the bq25892 set the IINLIM based on external charger detection")
Cc: stable@vger.kernel.org
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/supply/bq25890_charger.c

@@ -750,7 +750,7 @@ static void bq25890_charger_external_power_changed(struct power_supply *psy)
 	if (bq->chip_version != BQ25892)
 		return;
 
-	ret = power_supply_get_property_from_supplier(bq->charger,
+	ret = power_supply_get_property_from_supplier(psy,
 						      POWER_SUPPLY_PROP_USB_TYPE,
 						      &val);
 	if (ret)