tbf_fsm: Identify several events as Dl-TBF only

This will help in the future when splitting tbf_fsm into different FSMs
for UL-TBF and DL-TBF, since only some of the events and states are
shared.
That means we can keep a single state and event enum, but the FSMs can
be implemented separately in different files, easing a lot extending and
understanding the logic.

Change-Id: Id97f0d532d2d7ab2791a35c1f836d7c8da050124
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M src/tbf_fsm.c
1 file changed, 7 insertions(+), 0 deletions(-)