common/sfc_efx/base: handle Tx complete on Riverhead

common/sfc_efx/base: handle Tx complete on Riverhead

Introduce a new event callback which has the same prototype, but
provides number of completed descriptors instead of the last
completed descriptor index.

When all libefx-based drivers implement the new callback, libefx
may be updated to use it for Siena and EF10 family NICs and
the old one may be removed.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>