bts: fix pchan config in default mp_trx_pars[]
The timeslot 3 of transceiver 2 is configured to PDCH in osmo-bsc.cfg,
so the pchan config must be GSM_PCHAN_PDCH, not GSM_PCHAN_TCHF.
Change-Id: I7de4e2796ca91e90f9261be8355e97e60a9cc13f
Fixes: 42d8bd501 ("BTS_Tests: re-organize configuration of TRX/TS params")
