V4L/DVB (8731): zr364xx: remove BKL

Remove the Big Kernel Lock from zr364xx driver after pushdown.
Now using an internal locking mecanism on open().

Signed-off-by: Antoine Jacquet <royale@zerezo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>