netxen: fix infinite loop on dma mapping failure

Fix a perpetual while() loop in unwinding partial
mapped tx skb on dma mapping failure.

Reported-by: "Juha Leppanen" <juha_motorsportcom@luukku.com>
Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>