media: em28xx: fix memory leak in em28xx_init_dev

In the em28xx_init_rev, if em28xx_audio_setup fails, this function fails
to deallocate the media_dev allocated in the em28xx_media_device_init.

Fix this by adding em28xx_unregister_media_device to free media_dev.

BTW, this patch is tested in my local syzkaller instance, and it can
prevent the memory leak from occurring again.

CC: Pavel Skripkin <paskripkin@gmail.com>
Fixes: 37ecc7b1 ("[media] em28xx: add media controller support")
Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>