[media] ati_remote: allow specifying a default keymap selector function

Currently the ati_remote default keymap is selected directly based on
the USB device id.

Add support for instead specifying a function returning the default
keymap, allowing more complex selection logic to be added when needed.

This will be used for Medion X10 remotes in a following commit.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>