V4L/DVB: IR: Allow not to compile keymaps in

Currently, ir device registration fails if keymap requested by driver is not found.
Fix that by always compiling in the empty keymap, and using it as a failback.

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>