netxen: annotate dma watchdog setup

o remove superfluous code to setup PCI dma watchdog for NX2031.
o disable dma watchdog completely for NX3031 (not required).

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