scheduler: fix: use ts_pchan() in trx_sched_set_cipher()
scheduler: fix: use ts_pchan() in trx_sched_set_cipher()

Properly handle dynamic timeslots, do not access 'pchan' directly.

Change-Id: Iccb8794253face54164af1ba344204eadab8f98b
Fixes: [1] I7c4379e43a25e9d858d582a99bf6c4b65c9af481

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