V4L/DVB (5575): Pvrusb2: Implement ability to disable IR receiver

Anyone using multiple PVR USB2 devices really only want one of them
acting as the actual IR receiver.

Implemented here is a new per-instance module option (ir_mode) which is
a flag to enable the IR receiver.  The default is enabled.

IR reception is disabled by blocking access to the IR receiver chip in
the device.

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