6lowpan: duplicate definition of IEEE802154_ALEN

The same macros is defined in 'include/net/af_ieee802154.h' and is called
IEEE802154_ADDR_LEN. No need another one, so remove it.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>