tsnep: Add XDP TX support

Implement ndo_xdp_xmit() for XDP TX support. Support for fragmented XDP
frames is included.

Also some braces and logic cleanups are done in normal TX path to keep
both TX paths in sync.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>