media: meson: vdec: fix possible refcount leak in vdec_probe()

v4l2_device_unregister need to be called to put the refcount got by
v4l2_device_register when vdec_probe fails or vdec_remove is called.

Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>