NetXen: Fix compile failure seen on PPC architecture

NetXen: Add NETXEN prefixes to macros to clean them up.
This is a cleanup patch which adds NETXEN prefix to some stand
alone macro names.
These posed compile errors when NetXen driver was backported to 2.6.9
on PPC architecture as macros like USER_START are defined in file
arch/ppc64/mm/hash_utils.c

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off by: Wen Xiong <wenxiong@us.ibm.com>
Acked-off by: Mithlesh Thukral <mithlesh@netxen.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>