net: ethernet: fs_enet: convert to SPDX

The ENET driver has SPDX tags in the header files, but they were missing
in the C files. Change the licence information to SPDX format.

Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>