V4L/DVB: cx18: Rename snd_cx18_mixer_lock to snd_cx18_lock and increase visibility

Rename snd_cx18_mixer_lock() to snd_cx18_lock() in anticpation of using it
in the cx18-alsa-pcm.c file routines.

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