V4L/DVB: cx18-alsa: codingstyle cleanup

Move the cx18_alsa_announce_pcm_data() function further up in the file, since
apparently "make checkpatch" has never heard of a forward declaration.  Note
that despite the hg diff showing everything else as having been deleted/added,
in reality it was only that one function that got moved (and the forward
declaration was removed from the top of the file).

This work was sponsored by ONELAN Limited.

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