bonding: bond_update_speed_duplex() can return void since no callers check its return

As none of the callers of bond_update_speed_duplex (need to) check its
return value, there is little point in it returning anything.

Signed-off-by: Rick Jones <rick.jones2@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>