bonding: Fix another case of LACPDU not sent on slave

When mii-mon discovers that the link is up, it will call
bond_3ad_handle_link_change() but we forget to add the LACP_ENABLED
flag when we discover the speed and duplex for the slave link are
normal.

Change-Id: Ie8b268ecfeea0f99bf9fdcd72706c0653f9d9e49
Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>