board/esd/cpci750/mv_eth.c: Fix license

The file header indicated that this file was GPL-2.0+, but actually
the code was derived from (Marvell based) Linux source code which is
only GPL-2.0.  Fix this.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Acked-by: Stefan Roese <sr@denx.de>