netxen: refactor indirect register access

Refactor code to calculate and set indirect access
window for control registers in 2MB address space
(NX3031 or newer). Use void __iomem * data type for
absolute pci addresses.

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