media: platform: allgro-dvt: unregister v4l2_device on the error path

In allegro_probe(), the v4l2 device is not unregistered in the error
path, which results in a memory leak. Fix it by calling
v4l2_device_unregister() before returning error.

Fixes: d74d4e23 ("media: allegro: move driver out of staging")
Signed-off-by: Joe Hattori <joe@pf.is.s.u-tokyo.ac.jp>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>