net: fec: remove last_bdp from fec_enet_txq_xmit_frame()

The last_bdp is initialized to bdp, and both last_bdp and bdp are
not changed. That is to say that last_bdp and bdp are always equal.
So bdp can be used directly.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230529022615.669589-1-wei.fang@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>