replace magic tmsi constant with define

Depends: libosmocore I52b9f6b5f3e96d85a390ba2af21d7814df8aaeec
Change-Id: Icf9f466efce520779c926b47b6e6d6c9815120eb
diff --git a/src/gprs/gprs_sgsn.c b/src/gprs/gprs_sgsn.c
index 5801107..8d3de7b 100644
--- a/src/gprs/gprs_sgsn.c
+++ b/src/gprs/gprs_sgsn.c
@@ -692,12 +692,12 @@
 	 * Alternatively, a freeze list could be used if another PRNG is used
 	 * or when this approach proves to be not sufficient.
 	 */
-	if (ptmsi >= 0xC0000000) {
+	if (ptmsi >= GSM23003_TMSI_SGSN_MASK) {
 		if (!max_retries--)
 			goto failed;
 		goto restart;
 	}
-	ptmsi |= 0xC0000000;
+	ptmsi |= GSM23003_TMSI_SGSN_MASK;
 
 	if (ptmsi == GSM_RESERVED_TMSI) {
 		if (!max_retries--)