bonding: fix slave speed reporting in bond_miimon_commit

When we have BOND_LINK_UP the speed is reported unconditionally with %u
format although it can be SPEED_UNKNOWN (-1). After this patch it returns
0 in that case in an attempt to keep the existing scripts happy.
One line is intenionally left 81 chars because it gets ugly if broken.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Acked-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>