V4L/DVB: dvb/bt8xx: kill the big kernel lock

The bt8xx driver only uses the big kernel lock in its dst_ca_ioctl
function and never to serialize against other code, so we can
trivially replace it with a private mutex.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>