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>