fec: replace flush_dcache_range with dma_sync_single

flush_dcache_range is not portable across architectures. Use
dma_sync_single instead. Also, the memory must be synchronised in the
receive path aswell.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>