etherdevice: introduce eth_broadcast_addr

A lot of code has either the memset or an inefficient copy
from a static array that contains the all-ones broadcast
address. Introduce eth_broadcast_addr() to fill an address
with all ones, making the code clearer and allowing us to
get rid of some constant arrays.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>