osmo-bts-trx/scheduler: rx_rach_fn(): use optional TSC info from TRX
osmo-bts-trx/scheduler: rx_rach_fn(): use optional TSC info from TRX

TSC (Training Sequence Code) is an optional parameter of the UL burst
indication. We need this information in order to decide whether an
Access Burst is 11-bit encoded or not (see OS#1854).

If this information is absent, we try to correlate the received synch.
sequence with the known ones (3GPP TS 05.02, section 5.2.7), and
fall-back to the default TS0 if it fails.

Since the new TRXD header version, the training sequence code is
indicated by the transceiver. Let's use it!

Change-Id: I1e654a2e49cb83c5f1e6249c0de688f99bc466b0
Related: OS#1854, OS#4006

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