bonding: replace the return value type

The method ndo_start_xmit is defined as returning a
netdev_tx_t, which is a typedef for an enum type,
but the implementation in this driver returns an int.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>