Changes

PCUIF: fix usage of bts number in PCUIF_TXT_IND

When we receive the PCU_VERSION using tr_PCUIF_TXT_IND we must ignore the
included BTS number because the PCU_VERSION is not addressed to a specific
BTS. When we send a PCU_VERSION using ts_PCUIF_TXT_IND, we should always
use the bts number 0 to be consistent (the BSC/BTS will ignore this number
anyway).

Let's fix the usage of tr_PCUIF_TXT_IND and put comments, to make clear why
the above applies.

Change-Id: I93de261fc77806b2f817e0d30cb1b0d377ed0dbb
related: OS#6507