sched: Remove multi-level queue priority limit

Modified bitmask to  bitmask array, it can make multilevel queue remove
32 bit prioriry limit.

We can scan bitmask array to find which queue have ready thread.

Only need the number of queues as priority because the priority
is checked on create_thread.

Signed-off-by: TaiJu Wu <tjwu1217@gmail.com>