drivers/net/mpc512x_fec.c: Fix build warning

Fix:
mpc512x_fec.c: In function 'mpc512x_fec_initialize':
mpc512x_fec.c:634: warning: assignment from incompatible pointer type

Tested on mpc5121ads board using tftp and nfs commands.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Joe Hershberger <joe.hershberger@gmail.com>