netxen: Fix bug in Tx completion path.

o Driver is not updating sw_consumer while processing Tx completion
  when interface is going down. Due to this interface down path gets
  stuck forever waiting for NAPI to complete.

Signed-off-by: Manish Chopra <manish.chopra@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>