net: fec: Add a SPDX identifier

Currently there is no license information in the header of
this file.

The MODULE_LICENSE field contains ("GPL"), which means
GNU Public License v2 or later, so add a corresponding
SPDX license identifier.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>