bonding: show saner speed for broadcast mode

Broadcast mode bonds transmit a copy of all traffic simultaneously out of
all interfaces, so the "speed" of the bond isn't really the aggregate of
all interfaces, but rather, the speed of the slowest active interface.

Also, the type of the speed field is u32, not unsigned long, so adjust
that accordingly, as required to make min() function here without
complaining about mismatching types.

Fixes: bb5b052f ("bond: add support to read speed and duplex via ethtool")
CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: "David S. Miller" <davem@davemloft.net>
CC: netdev@vger.kernel.org
Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>