bonding: fix wrong port enabling in 802.3ad

The port shouldn't be enabled unless its current MUX
state is DISTRIBUTING which is correctly handled by
ad_mux_machine(), otherwise the packet sent can be
lost because the other end may not be ready.

The issue happens on every port initialization, but
as the ports are expected to move quickly to DISTRIBUTING,
it doesn't cause much problem.  However, it does cause
constant packet loss if the other peer has the port
configured to stay in STANDBY (i.e. SYNC set to OFF).

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>