Merge bts_alloc_ms() and ms_alloc()

gprs_default_cb_ms_idle() is changed to have the same implementation as
previous bts_ms_idle_cb(), since that's the only one being used in
osmo-pcu code. It makes no sense to use different callback logic in unit
tests.

This is another step towards simplifying the code and getting rid of the
idle/active_cb().

Change-Id: I2a06d17588572a21dc5a14ddbde83766076b446d
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index cca059a..cdd703c 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -216,7 +216,7 @@
 	}
 
 	if (!ms)
-		ms = bts_alloc_ms(bts);
+		ms = ms_alloc(bts);
 	if (imsi)
 		ms_set_imsi(ms, imsi);
 	ms_confirm_tlli(ms, tlli);