6lowpan: revert 'reuse eth_mac_addr()'

This reverts the commit cdf49c28

 which
replaces lowpan '.ndo_set_mac_address' method by ethernet's one.

Accorind to the IEEE 802.15.4 standard, device has 8-byte length address,
so this hook loses the last 2 bytes which may rise a compatibility problems
with other IEEE 802.15.4 standard implementations.

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