net: sgi: ioc3-eth: fix setting NETIF_F_HIGHDMA

Set NETIF_F_HIGHDMA together with the NETIF_F_IP_CSUM flag instead of
letting the second assignment overwrite it.  Probably doesn't matter
in practice as none of the systems an IOC3 is usually found in has
highmem to start with.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>