move BTS-specific timezone override into sub-structure

Group all three structure members involved in bts-specific timezone
handling into a sub-structure.
diff --git a/openbsc/src/libmsc/gsm_04_08.c b/openbsc/src/libmsc/gsm_04_08.c
index 9bdc39c..c868d7c 100644
--- a/openbsc/src/libmsc/gsm_04_08.c
+++ b/openbsc/src/libmsc/gsm_04_08.c
@@ -717,18 +717,18 @@
 	ptr8[5] = bcdify(gmt_time->tm_min);
 	ptr8[6] = bcdify(gmt_time->tm_sec);
 
-	if (bts->tz_bts_specific) {
-		/* Convert tzhr and tzmn to units */
-		if (bts->tzhr < 0) {
-			tzunits = ((bts->tzhr/-1)*4);
-			tzunits = tzunits + (bts->tzmn/15);
+	if (bts->tz.override) {
+		/* Convert tz.hr and tz.mn to units */
+		if (bts->tz.hr < 0) {
+			tzunits = ((bts->tz.hr/-1)*4);
+			tzunits = tzunits + (bts->tz.mn/15);
 			ptr8[7] = bcdify(tzunits);
 			/* Set negative time */
 			ptr8[7] |= 0x08;
 		}
 		else {
-			tzunits = bts->tzhr*4;
-			tzunits = tzunits + (bts->tzmn/15);
+			tzunits = bts->tz.hr*4;
+			tzunits = tzunits + (bts->tz.mn/15);
 			ptr8[7] = bcdify(tzunits);
 		}
 	}