netxen: handle dma mapping failures

o Bail out if pci_map_single() fails while replenishing rx ring.
o Drop packet if pci_map_{single,page}() fail in tx.

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>