i40e: update Shadow RAM read/write functions

This change is to refactor the read/write functions to support
future work.

Change-ID: I13150d5e3042f2c617362c0140dc7e6473ebcdee
Signed-off-by: Kamil Krawczyk <kamil.krawczyk@intel.com>
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>