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