IEEE 802.15.4: do not enable driver debugging by default

The IEEE 802.15.4 drivers were compiled by default with debugging,
which caused them to be rather chatty and slow. This patch silences
them. People debugging drivers can still add a #define DEBUG in the
beginning of the respective file or use dynamic debug

This patch also removes the now unused option CONFIG_FFD.

Signed-off-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>