V4L/DVB: cx18: export a couple of symbols so they can be shared with cx18-alsa

Expose a couple of symbols in the cx18 module so that locking of the PCM
stream can be shared with the cx18-alsa module.

This work was sponsored by ONELAN Limited.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>