i40e/i40evf: napi_poll must return the work done

Currently the function i40e_napi-poll() returns 0 when it clean completely
the Rx rings, but this foul budget accounting in core code.

Fix this by returning the actual work done, capped to budget - 1, since
the core doesn't allow to return the full budget when the driver modifies
the NAPI status

This is based on a similar change that was made for the ixgbe driver by
Paolo Abeni.

Change-ID: Ic3d93ad2fa2fc8ce3164bc461e69367da0f9173b
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_txrx.c

@@ -2027,7 +2027,7 @@ int i40e_napi_poll(struct napi_struct *napi, int budget)
 	else
 		i40e_update_enable_itr(vsi, q_vector);
 
-	return 0;
+	return min(work_done, budget - 1);
 }
 
 /**

drivers/net/ethernet/intel/i40evf/i40e_txrx.c

@@ -1490,7 +1490,7 @@ int i40evf_napi_poll(struct napi_struct *napi, int budget)
 	else
 		i40e_update_enable_itr(vsi, q_vector);
 
-	return 0;
+	return min(work_done, budget - 1);
 }
 
 /**