V4L/DVB (6949): Adds em28xx-audio module

em28xx-audio module exports em28xx Vendor Class audio as an -alsa
driver. This module were written based on usbaudio driver by Markus
Rechberger. Recently, he acked to allow us to merge it on kernel:

http://lists-archives.org/video4linux/20408-supporting-prolink-pixelview-405-dvd-maker.html

Thanks to Markus Rechberger <mrechberger@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>