V4L/DVB (12600): dvb-usb-remote: return KEY_RESERVED if there's free space for new keys

The input subsystem checks if get_keycode works for a scan code. Due to that,
we need to return a valid value when there's some space at the table that
can be used by a scancode.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>