Fix gsm_pchan2chan_nr() to use RSL_CHAN_OSMO_PDCH

When converting from GSM_PCHAN_PDCH, we should generate
a RSL channel number IE with the osmocom extension
RSL_CHAN_OSMO_PDCH rather than claiming it is a regular
TCH/F channel.

This is important as this function is used by
osmo-bts, too - and it decides which channel number IE is
put in the GSMTAP header for both GSMTAP tracing as well
as the GSMTAP based osmo-bts-virtual.

In order to avoid any unintended effect on libbsc,
we make sure to modify rsl_ipacc_pdch_activate() to
always use GSM_PCHAN_TCH_F in related RSL message.

Change-Id: Ie34219e64a6d89da4a79f2db8ec73d1909fb8280
diff --git a/src/libcommon/gsm_data_shared.c b/src/libcommon/gsm_data_shared.c
index d792f3b..2696273 100644
--- a/src/libcommon/gsm_data_shared.c
+++ b/src/libcommon/gsm_data_shared.c
@@ -644,11 +644,14 @@
 
 	switch (pchan) {
 	case GSM_PCHAN_TCH_F:
-	case GSM_PCHAN_PDCH:
 	case GSM_PCHAN_TCH_F_PDCH:
 		OSMO_ASSERT(lchan_nr == 0);
 		cbits = 0x01;
 		break;
+	case GSM_PCHAN_PDCH:
+		OSMO_ASSERT(lchan_nr == 0);
+		cbits = RSL_CHAN_OSMO_PDCH >> 3;
+		break;
 	case GSM_PCHAN_TCH_H:
 		OSMO_ASSERT(lchan_nr < 2);
 		cbits = 0x02;