[media] cx25821, cx88, tm6000: use SNDRV_DEFAULT_ENABLE_PNP

Instead of manually initializing the bool array enable, use the
SNDRV_DEFAULT_ENABLE_PNP macro. As most drivers do.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>