sfc: Replace memset with eth_zero_addr

Use eth_zero_addr to assign zero address to the given address array
instead of memset when the second argument in memset is address
of zero which makes the code clearer and also add header
file linux/etherdevice.h

Signed-off-by: Shyam Saini <mayhs11saini@gmail.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>