[core/sched/cbs] Bugfix numerical error on signed comparison

The lower bound of a round is computed by the difference between a singed
and an unsigned. Force this subtraction to be a signed number before
to proceed with the comparison.

Without this patch the condition:
  cbs_rq->round_tq_corr_old < (burst_tq_lower_bound - cbs_rq->round_tq)
was true even for the case:
  (s64)4765 < ((s64)1500 - (u32)6006)

Signed-off-by: Patrick Bellasi <derkling@gmail.com>
[core/sched/cbs] Rename rounds upper and lower bounds

The so far named burst_tq_[lower|upper]_bound are actually boundaries for
the round correction value. Thus this patch rename them just to better
match their meaning.

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