bsc: Add define for ts_as_pchan_for_each_lchan with ts->pchan_on_init

It will be used further in follow-up patches. It also provides a place
to document its (intricate) logic around it and its possible uses.

Change-Id: Ia1d4bdbfca6b9719f54ee609b6bfadf7f3a4bb43
2 files changed