libmsc/gsm_04_11.c: fix: always use SAPI 3 for MT SMS

In I4a07ece80d8dd40b23da6bb1ffc9d3d745b54092 I've introduced a
regression. According to GSM TS 04.11, section 2.3, SAPI 3 shall
be used for both MO/MT SMS transmissions. Due to a mistake,
caused by misunderstanding of the meaning of trans->dlci, SAPI 3
was not assigned to SM transactions if there is already an active
RAN connection with subscriber. Let's fix this.

Let's also drop this misleading comment:

  /* FIXME: specify SACCH in case we already have active TCH */

because it's a task of the BSC/BTS to decide which lchan to use.

Change-Id: I08d0801a89d377441e95fb8e3dd27c8d587f89e9
Related: OS#3716
1 file changed