bonding: fix LACP PDU not sent on slave port sometimes

When a slave is added to a bond and it is not in full duplex mode,
AD_PORT_LACP_ENABLED flag is cleared, due to this LACP PDU is not sent
on slave. When the duplex is changed to full, the flag needs to be set
to send LACP PDU.

Cc: Andy Gospodarek <gospo@cumulusnetworks.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: Satish Ashok <sashok@cumulusnetworks.com>
Signed-off-by: Jonathan Toppins <jtoppins@cumulusnetworks.com>
Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>