[PATCH] mv643xx_eth: Unmap DMA buffers in receive path

Fix a missing call to dma_unmap_single() in the receive path.  Without
this call, errors have been observed on non-cache-coherent systems.

Signed-off-by Dale Farnsworth <dale@farnsworth.org>

Signed-off-by: Jeff Garzik <jeff@garzik.org>