bonding: Introduce 4 AD link speed to fix agg_bandwidth

This patch adds [2.5|20|40|56] Gbps enum definition, and fixes
aggregated bandwidth calculation based on above slave links.

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: David S. Miller <davem@davemloft.net>
Signed-off-by: Jianhua Xie <jianhua.xie@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>