net/fec: put the ioremap cookie immediately into a void __iomem pointer

Saving it first into struct net_device->base_addr (which is an unsigned
long) is pointless and only needs to use more casts than necessary.

Reported-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>