bsc_api: Make sure to use correct MS Power on ASSIGNMENT CMD

When gsm48_send_rr_ass_cmd() is being called to send the ASSIGNMENT
COMMAND, we need to use the propwer lchan->ms_power setting, not
some fixed magic "0x3" number.

Without this patch, every MS would transmit at a very high output power
fullowing an assignment command - more than what was set in the config
file with "ms max power"
diff --git a/openbsc/src/libbsc/bsc_api.c b/openbsc/src/libbsc/bsc_api.c
index 8ddbc6c..9504e53 100644
--- a/openbsc/src/libbsc/bsc_api.c
+++ b/openbsc/src/libbsc/bsc_api.c
@@ -665,7 +665,7 @@
 		return;
 
 	LOGP(DMSC, LOGL_NOTICE, "Sending assignment on chan: %p\n", lchan);
-	gsm48_send_rr_ass_cmd(conn->lchan, lchan, 0x3);
+	gsm48_send_rr_ass_cmd(conn->lchan, lchan, lchan->ms_power);
 }
 
 static void handle_chan_nack(struct gsm_subscriber_connection *conn,