bonding: fix 802.3ad support for 5G and 50G speeds

This patch adds [5|50] Gbps enum definition, and fixes
aggregated bandwidth calculation based on above slave links.

Fixes: c9a70d43 ("net-next: ethtool: Added port speed macros.")
Signed-off-by: Thibaut Collet <thibaut.collet@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>