sch_cake: drop unused variable tin_quantum_prio

Turns out tin_quantum_prio isn't used anymore and is a leftover from a
previous implementation of diffserv tins.  Since the variable isn't used
in any calculations it can be eliminated.

Drop variable and places where it was set.  Rename remaining variable
and consolidate naming of intermediate variables that set it.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>