osmo-bts-trx/trx_if.c: encode actual TRXD header version
osmo-bts-trx/trx_if.c: encode actual TRXD header version

Both TRX2L1 (Uplink) and L12TRX (Downlink) messages should use
the same TRXD header format (and version) as was negotiated.

Change-Id: Idbc598ef7c1871ee8da830f3fbe0a5cc386f873d
Related: OS#4006

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