bsc: Send MS Power Control msg upon max MS power change

Lots of times, the MS power class is unknown until after the first
channel has been activated, at which point the MS power class is
received in messages such as LU update or CM Service Requet.
Since the MS Power level is sent upon CHAN ACT, the only way to
communicate the change of maximum MS Power (based on MS power class)
after CHAN ACT is to send a MS Power Control msg. Let's do that.

Related: OS#4244
Change-Id: I3d6b75578e5cb9b2ad474a0ad01362d846ebe135
diff --git a/tests/bsc/bsc_test.c b/tests/bsc/bsc_test.c
index 103e0bb..86b7282 100644
--- a/tests/bsc/bsc_test.c
+++ b/tests/bsc/bsc_test.c
@@ -252,3 +252,4 @@
 void lchan_activate(struct gsm_lchan *lchan, void *info) {}
 bool neighbor_ident_bts_entry_exists(uint8_t from_bts) { return false; }
 const char *handover_status(struct gsm_subscriber_connection *conn) { return "x"; }
+int rsl_chan_ms_power_ctrl(struct gsm_lchan *lchan) { return 0; }