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.

This patch does not guard against sporadic gaps in the Downlink TCH
queue.  However when tch_dl_dequeue() constantly yields nothing, e.g.
when we end up with a codec mismatch, then this additional check saves
us from starting misaligned FACCH/H transmission.  If we start at a wrong
TDMA offset, then all subsequent FACCH frames will be scheduled at wrong
offsets and thus none of them will be decoded by the MS.

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(-)