net: sgi: ioc3-eth: simplify setting the DMA mask

There is no need to fall back to a lower mask these days, the DMA mask
just communicates the hardware supported features.

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>