i40e: limit debugfs io ops

Don't let the debugfs register read and write commands try to access
outside of the ioremapped space.  While we're at it, remove the use of
a misleading constant.

Change-ID: Ifce2893e232c65c7a76c23532c658f298218a81b
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>