net/6lowpan: Remove FSF address from GPL statement.

This might change and we already deliver a copy of the license with
the kernel. This was already removed form the ieee802154 code but
missed here.

Signed-off-by: Stefan Schmidt <s.schmidt@samsung.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>