V4L/DVB (4825): FIX bug 5760: audio were not working on some bttv drivers

This fixes a bug introduced by the -git commit:
	bbf7871e

It seems that some bttv apps can't work fine when audioset=0.
Thanks to Christian Casteyde <casteyde.christian@free.fr> for pointing this.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>