netxen: fix minor tx timeout bug

Fix minor bug in netdev tx timeout handling which could
always lead to firmware reset instead of pci function reset.

netxen_nic_reset_context() requires __NX_RESETTING bit
cleared.

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