bonding: warn user when 802.3ad speed is unknown

Goal is to advertise the user when ethtool speeds and 802.3ad speeds are
desynchronized.
When this case happens, the kernel needs to be patched.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>