ixgbe: napi_poll must return the work done

Currently the function ixgbe_poll() returns 0 when it clean completely
the rx rings, but this foul budget accounting in core code.
Fix this returning the actual work done, capped to weight - 1, since
the core doesn't allow to return the full budget when the driver modifies
the napi status

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Venkatesh Srinivas <venkateshs@google.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>