benet: remove broken and unused macro

is_broadcast_packet() expands to compare_ether_addr() which doesn't
exist since commit 7367d0b5

 ("drivers/net: Convert uses of
compare_ether_addr to ether_addr_equal"). It turns out it's actually not
used.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: David S. Miller <davem@davemloft.net>