bonding: use last_arp_rx in slave_last_rx()

Now that last_arp_rx really has the last time we've received any (validated or
not) ARP, we can use it in slave_last_rx() instead of slave->dev->last_rx.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>