[netdrvr pcnet32] add missing pci_dma_sync_single

[netdrvr pcnet32] add missing pci_dma_sync_single

a patch for the pcnet32.c driver which adds a missing call to
pci_dma_sync_single.  If a received packet is smaller than rx_copybreak
the pcnet driver will recycle the receive buffer which requires calling
pci_dma_sync_single.  Patch is against 2.6 but I it's also needed in 2.4.

Without that call the processor might still have old stale data in
the data cache when the processor accesses the recycled buffer.