V4L/DVB: cx18: Add missing serialization locking to cx18-alsa

File operations that manipulate the state of the encoder, or video or audio
subdevices or some of the stream flags need to be serialized.  Add the
serilization locks to cx18-alsa in a manner consistent with the locking for
the v4l2 file operations.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>