i40e: Reduce range of interrupt reg in reg test

Use a smaller range of test registers in MFP mode as there are fewer
resources than when in SFP mode.

Change-Id: I08424890c3f57b5dde5ee99e99724ce252e0875a
Signed-off-by: Kamil Krawczyk <kamil.krawczyk@intel.com>
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>