net: rswitch: Fix type of ret in rswitch_start_xmit()

The type of ret in rswitch_start_xmit() should be netdev_tx_t. So,
fix it.

Fixes: 3590918b

 ("net: ethernet: renesas: Add support for "Ethernet Switch"")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>