net/ieee802154/6lowpan.c: reuse eth_mac_addr()

Use eth_mac_addr() for .ndo_set_mac_address, remove
lowpan_set_address since it do currently the same as
eth_mac_addr(). Additional advantage: eth_mac_addr() already
checks if the given address is valid

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Acked-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>