Input: adc-joystick - fix ordering in adc_joystick_probe()

We should register the IIO buffer before we register the input device,
because as soon as the device is registered input handlers may attach to
it, resulting in a call to adc_joystick_open() which makes use of the said
buffer.

Acked-by: Artur Rojek <contact@artur-rojek.eu>
Link: https://lore.kernel.org/r/YskFh4NHnlcryMkk@google.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>