ubsan: fix shift

Ubsan complains about shifts into the sign bit due to automatic int
promotion, so cast explicitly.

Change-Id: I6387c7313832f6c7c920e1016b74562b66d6b68e
Related: OS#4029
diff --git a/src/bts.cpp b/src/bts.cpp
index 395fc73..26dd401 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -551,7 +551,7 @@
 	}
 
 	/* get TLLI from downlink assignment */
-	tlli = ((*data++) & 0xf) << 28;
+	tlli = (uint32_t)((*data++) & 0xf) << 28;
 	tlli |= (*data++) << 20;
 	tlli |= (*data++) << 12;
 	tlli |= (*data++) << 4;