bts: Rename bts_ms_by_{tlli,imsi} -> bts_get_ms_by_{tlli,imsi}

While at it, put them together and mark bts param as const.
This is a preparation for next patch.

Change-Id: Iad8aec4424f1f23cd4d02a14c4f9ec1b9fdb1f75
diff --git a/src/gprs_ms.c b/src/gprs_ms.c
index 9ce3d82..909e7f8 100644
--- a/src/gprs_ms.c
+++ b/src/gprs_ms.c
@@ -558,7 +558,7 @@
 		"Modifying MS object, TLLI = 0x%08x, IMSI '%s' -> '%s'\n",
 		ms_tlli(ms), ms->imsi, imsi);
 
-	struct GprsMs *old_ms = bts_ms_by_imsi(ms->bts, imsi);
+	struct GprsMs *old_ms = bts_get_ms_by_imsi(ms->bts, imsi);
 	/* Check if we are going to store a different MS object with already
 	   existing IMSI. This is probably a bug in code calling this function,
 	   since it should take care of this explicitly */