i40e: check MAC type before any REG access

We need to check if we are dealing with the correct MAC type before we
try to read anything from the registers.

Change-Id: I3989516999d06c3009e87d6a2eafc20af305c5c2
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>