8139cp: balance dma_map_single vs dma_unmap_single pair

The driver always:
1. allocate cp->rx_buf_sz + NET_IP_ALIGN
2. map cp->rx_buf_sz

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>