carl9170 firmware: add note about rx filter usage

This patch adds a note about what it means when
a rx filter bit is set. Because unlike what most
people assume, it means that the data which matches
the condition is discarded and not
"passed to the host".

Reported-by: Javier Lopez <jlopex@cozybit.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>