lan743x: sync only the received area of an rx ring buffer

On cpu architectures w/o dma cache snooping, dma_unmap() is a
is a very expensive operation, because its resulting sync
needs to invalidate cpu caches.

Increase efficiency/performance by syncing only those sections
of the lan743x's rx ring buffers that are actually in use.

Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Reviewed-by: Bryan Whitehead <Bryan.Whitehead@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>