osmo-bts-trx/scheduler: refactor dummy burst scheduling

This change facilitates the upcoming freq. hopping implementation,
in particular scheduling of dummy bursts on C0 with hopping time-
slots.  One problem is that we cannot know in advance, whether to
send a dummy burst on a given timeslot unless all transceivers are
processed.  For example, trx#3 may want to send a normal burst on
ARFCN of trx#0 (C0), while we have already sent a dummy burst...

Another important aspect is that we shall not be sending dummy
bursts on transceivers other than C0.  Scheduling dummy bursts
from _sched_dl_burst() in the context of a single hopping timeslot
of a single transceiver leaves trx_sched_fn() no way to know
whether it's a dummy burst or something else.

Let's solve both problems by moving dummy burst scheduling logic
from _sched_dl_burst() to trx_sched_fn().  Maintain C0 slot-mask
in the inner (per-trx) loop, so that we can fill missing bursts
with dummy bursts afterwards.

Change-Id: I8c3651c27d2991079e83b8abdb7e2c3f95b97a43
Related: SYS#4868, OS#4546

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