trx_sched_is_sacch_fn(): fix handling of dynamic timeslots
trx_sched_is_sacch_fn(): fix handling of dynamic timeslots

There is no GSM_PCHAN_TCH_F{_TCH_H}_PDCH in trx_sched_multiframes[],
so find_sched_mframe_idx() would always return -1 for these.  Fix
this by using ts_pchan(), which returns currently active pchan type.

Change-Id: Ia5e337e897b595e7de6e504664c969b583cc02a1

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