drivers: net: xgene: fix: Add dma_unmap_single

In addition to xgene_enet_delete_bufpool() freeing skbs, their associated
dma memory should also be unmapped.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>