V4L/DVB (6952): Add code for autoloading em28xx-alsa, if needed

Older em28xx devices does implement standard Audio Class.

However, on newer devices, this were replaced by a Vendor Class. This
patch autodetects that an em28xx lacks Audio Class and auto-loads
em28xx-alsa, for the devices that implements only a Vendor Class.

For devices with Audio Class, snd-usb-audio module will provide an ALSA
interface.

This patch uses the request_module_async function as defined on cx88-mpeg.c, 
originally wrote by Markus Rechberger.

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