octphy: activate CBCH after all physical channels are activated
octphy: activate CBCH after all physical channels are activated

CBCH is activated when the SAPI for TS0 is activated. Since the CBCH can be
configured on any physical TS, we wait until all (TS7 is the last) physical
timeslots are configured.

Change-Id: Ie307bf9f370a346686e3bd8c8a8483953a1bc279

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts