netxen:fix napi intr enable check

o netif_running() check for enabling interrupt at end of napi poll is
  not enough to cover firmwar recovery. Instead test __NX_DEV_UP bit.
o Avoid re-entry into to netxen_nic_down() with __NX_DEV_UP bit check.

Acked-by: Dhananjay Phadke <dhananjay.phadke@qlogic.com>
Signed-off-by: Amit Kumar Salecha <amit.salecha@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>