scheduler.c: Factor out find_sched_mframe_idx() function
scheduler.c: Factor out find_sched_mframe_idx() function

Let's split the look-up of the multiframe scheduler from the asignment
to a given l1ts in trx_sched_set_pchan.

Related: OS#2978
Change-Id: I79548b25aae647ce993a9d83c771d22b08cb1c74

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