[media] bt8xx: remove needless module refcounting

It is responsibility of a caller of fops->open(),
to make sure an owner of the fops is available until file is closed.
So, there is no need to lock THIS_MODULE explicitly.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>