staging: fsl-dpaa2/eth: Fix return type of ndo_start_xmit

ndo_start_xmit() returns a value of type netdev_tx_t. Update
our ndo function to use the correct type.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>