i40e: expose debug_write_register request

Now that the HW registers are no longer in debug mode and many are
locked down for writes, we need to expose the Firmware API request
used to do writes on the driver's behalf.

Change-ID: I09a05c4dc9ea0b24c00193faac34d7799eaa8496
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Jim Young <jamesx.m.young>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>