[core/sched] Bugfix 887ea1c0 - CBS sched class assignment

The CBS scheduler class must have precedence over the entity priority.
Without such a code path, settings a CBS class via chrt could lead to a
wrong assignment to the RT scheduler, e.g.
# chrt -c 0 hackbench
would lead to hackbench running as a 0 priority RT task.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>