netxen: pre calculate register addresses

For registers accessed in fast path (interrupt / softirq)
avoid expensive I/O address translation. These registers
are directly mapped in PCI bar 0 and do not require
any window checks.

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