osmo-bts-trx: check if scheduling of [dummy] FACCH/H is allowed

Currently (without this patch) if tch_dl_dequeue() yields nothing we're
scheduling dummy FACCH/H *regardless* if it's permitted to start at the
given TDMA FN or not.  This may result in misaligned FACCH transmission,
so the MS will not able to decode anything and will report BER>0.

With this patch applied we a) schedule FACCH if it's allowed to start at
the current TDMA FN;  b) send whatever was in the burst buffer (garbage)
if FACCH/H is not permitted.  This is the best we can do without
introducing additional complexity.  Ideally the upper layers should
guarantee that the queue is never empty, but currently this is not the
case.

Change-Id: I6f8af140a6ccf3d5fd7b98f6cb5c18e2c5e2f61b
Related: SYS#5919, OS#4823
---
M src/common/scheduler.c
M src/osmo-bts-trx/sched_lchan_tchh.c
2 files changed, 9 insertions(+), 1 deletion(-)