driver/net/benet: fix be_cmd_multicast_set() memcpy bug

Regarding  benet be_cmd_multicast_set() function, now using
netdev_for_each_mc_addr() helper for mac address copy, but
when copying to req->mac[] did not increase of the index.

Cc: Sathya Perla <sathyap@serverengines.com>
Cc: Subbu Seetharaman <subbus@serverengines.com>
Cc: Sarveshwar Bandi <sarveshwarb@serverengines.com>
Cc: Ajit Khaparde <ajitk@serverengines.com>
Signed-off-by: Joe Jin <joe.jin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>