gpiolib: Fix triggering "kobject: 'gpiochipX' is not initialized, yet" kobject_get() errors

When a gpiochip gets added by loading a module, then another driver may
be waiting for that gpiochip to load on the deferred-probe list.

If the deferred-probe for the consumer of gpiochip then triggers between
the gpiodev_add_to_list_unlocked() calls which makes gpio_device_find()
see the chip and the gpiochip_setup_dev() later then gpio_device_find()
does a kobject_get() on an uninitialized kobject since the kobject is
initialized by gpiochip_setup_dev() calling device_initialize():

 arizona spi-10WM5102:00: cannot find GPIO chip arizona, deferring
 arizona spi-10WM5102:00: cannot find GPIO chip arizona, deferring
 ------------[ cut here ]------------
 kobject: 'gpiochip5' (00000000241466f2): is not initialized, yet kobject_get() is being called.
 WARNING: CPU: 3 PID: 42 at lib/kobject.c:640 kobject_get+0x43/0x70
 Call Trace:
  kobject_get
  gpio_device_find
  gpiod_find_and_request
  gpiod_get
  snd_byt_wm5102_mc_probe

Not only is the device not initialized yet, but when the gpio-device is
added to the list things like the irqchip also have not been initialized
yet.

So gpio_device_find() should really ignore the gpio-device until
gpiochip_add_data_with_key() is fully done. Add a device_is_registered()
check to gpio_device_find() to ignore gpio-devices on the list which are
not yet fully initialized.

Fixes: aab5c6f2

 ("gpio: set device type for GPIO chips")
Suggested-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
[Bartosz: fix a typo in commit message]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>