sch_cake: Simplify logic in cake_select_tin()

[ Upstream commit 4976e3c683f328bc6f2edef555a4ffee6524486f ]

The logic in cake_select_tin() was getting a bit hairy, and it turns out we
can simplify it quite a bit. This also allows us to get rid of one of the
two diffserv parsing functions, which has the added benefit that
already-zeroed DSCP fields won't get re-written.

Suggested-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>