V4L/DVB (11927): em28xx: provide module option to disable USB speed check

Add an em28xx module option that allows a user to override the USB speed check.
Intended for advanced users who understand the consequences of trying to use
the device with a 12Mbps bus.

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