V4L/DVB (11923): em28xx: Don't let device work unless connected to a high speed USB...

V4L/DVB (11923): em28xx: Don't let device work unless connected to a high speed USB port

The em28xx basically just doesn't work at 12 Mbps.  The isoc pipe needs
nearly 200 Mbps for analog support, so users would see garbage video, and on
the DVB/ATSC side scanning is likely to work but if the user tried to tune it
would certainly appear to have failed.

It's better to fail explicity up front and tell the user to plug into a USB 2.0
port, than to let the driver load and the user have weird problems with tuning
and garbage video.

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