netxen: improve pci memory access

o Access on card memory through memory controller (agent)
  rather than moving small pci window around. Clean up the
  code for moving windows around.

o Restrict memory accesss to 64 bit, currently only firmware
  download uses this.

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