netxen: reduce stack usage of netxen_nic_flash_print

Don't need to keep a struct netxen_new_user_info on the stack
when we only are interested in printing the serial_num.  Change
to only reading the serial_num.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>