power_supply: Fix use after free and memory leak

device_unregister() might free its argument.  This leads to freed
memory use in kfree().  Also use put_device() instead of kfree()
as dev may be already used in another layer after call to device_add().

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>