ieee802154: add address struct with proper endiannes and some operations

Add a replacement ieee802154_addr struct with proper endianness on
fields. Short address fields are stored as __le16 as on the network,
extended (EUI64) addresses are __le64 as opposed to the u8[8] format
used previously. This disconnect with the netdev address, which is
stored as big-endian u8[8], is intentional.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>