[media] rc: rc-loopback: Add loopback of filter scancodes

Add the s_wakeup_filter callback to the rc-loopback driver, which instead
of setting the filter just feeds the scancode back through the input
device so that it can be verified.

Signed-off-by: James Hogan <james@albanarts.com>
Signed-off-by: Antti Seppälä <a.seppala@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Cc: David Härdeman <david@hardeman.nu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>