r8169: improve DMA handling in rtl_rx

Move the call to dma_sync_single_for_cpu after calling napi_alloc_skb.
This avoids calling dma_sync_single_for_cpu w/o handing control back
to device if the memory allocation should fail.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>