Documentation/networking/ieee802154.txt: fix various inaccuracies.

* Update the linux-zigbee git:// repository URL.

* Remove the MLME section as the current kernel does not provide a
  full 802.15.4 MLME implementation.

* The hardmac example driver 'fakehard' was removed some time ago.

* The IEEE 802.15.4 device drivers live in drivers/net/ieee802154/,
  not in drivers/ieee802154/.

* The IEEE 802.15.4 MTU is 127 bytes, not 128 bytes.

* Some of the 6LoWPAN code lives in net/6lowpan/.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>