net: fec: check DMA addressing limitations

Check DMA addressing limitations as suggested by the DMA API
how-to. This does not fix a particular issue seen but is
considered good style.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>