V4L/DVB (10122): em28xx: don't load em28xx-alsa for em2870 based devices

Like the em2874, the em2870 does not have any analog support, so don't bother
loading the em28xx-alsa module.

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