netxen: fix irq tear down and msix leak.

o Fix the order of irq and hardware context teardown.
  Also synchronize the interrupt in dev close() before
  releasing tx buffers.

o Fix possible msi-x vector leak if available vectors are
  less than requested.

o Request multiple msix vectors only if hardware supports
  multiple rx queues.

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