i40e: limit debugfs io ops

i40e: limit debugfs io ops

Orabug: 22342532

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>
(cherry picked from commit 2ac8b675d9466b397f94aa1ead575d39b71b5055)
Signed-off-by: Brian Maly <brian.maly@oracle.com>