net/sfc: factor out libefx-based Tx datapath

net/sfc: factor out libefx-based Tx datapath

Split control and datapath to make datapath substitutable and
possibly reusable with alternative control path.

libefx-based Tx datapath is bound to libefx control path, but
it should be possible to use other datapaths with alternative
control path(s).

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>