i40e: Rework register diagnostic

Register range, being subject to register diagnostic, can vary among
different NVMs. We will try to identify the full range and use it for
a register test. This is needed to avoid false test results. If we fail
to define the proper register range we will test only the first register
from that group.

Change-ID: Ieee7173c719733b61d3733177a94dc557eb7b3fd
Signed-off-by: Kamil Krawczyk <kamil.krawczyk@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>