V4L/DVB (7313): pvrusb2: Make LED control into a device-specific attribute

The pvrusb2 driver has used hardcoded logic to control the LED on the
device.  However this is really Hauppauge-specific behavior.  This
change defines a new device attribute for LED control and sets things
up appropriately for Hauppauge devices.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>