net/mlx4_en: Fix compilation error when CONFIG_INET isn't defined

ip_eth_mc_map function can't be used when CONFIG_INET isn't defined.
Fixed compilation error by adding CONFIG_INET define check before using the
function.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>