power_supply: fix return value of get_property

power_supply_get_property() should ideally return -EAGAIN if it is
called while the power_supply is being registered. There was no way
previously to determine if use_cnt == 0 meant that the power_supply
wasn't fully registered yet, or if it had already been unregistered.

Add a new boolean to the power_supply struct to simply show if
registration is completed. Lastly, modify the check in
power_supply_show_property() to also ignore -EAGAIN when so it
doesn't complain about not returning the property.

Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

include/linux/power_supply.h

@@ -248,6 +248,7 @@ struct power_supply {
 	struct delayed_work deferred_register_work;
 	spinlock_t changed_lock;
 	bool changed;
+	bool initialized;
 	atomic_t use_cnt;
 #ifdef CONFIG_THERMAL
 	struct thermal_zone_device *tzd;