V4L/DVB (9588): Don't load em28xx audio module for digital-only devices

Rework the logic so that the em28xx-alsa module does not get loaded for devices
that don't support analog audio (such as the em2874)

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>