bonding: replace SLAVE_IS_OK() with bond_slave_can_tx()

They're verifying the same thing (except of IFF_UP, which is implied for
netif_running(), which is also a prerequisite).

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>