[media] bttv-input: Add a note about PV951 RC

When comparing lirc-i2c and ir-kbd-i2c/bttv-input logic, a difference
was noticed. As lirc-i2c will be removed soon, store the difference on
a comment inside ir-kbd-i2c, in order to preserve the knowledge we
have about that remote controller.

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