export the 'trx_is_usable() function and use it from paging

this ensures we don't send paging requests to currently inactive transceivers.
diff --git a/openbsc/src/paging.c b/openbsc/src/paging.c
index 820773a..b5dc2d0 100644
--- a/openbsc/src/paging.c
+++ b/openbsc/src/paging.c
@@ -265,6 +265,11 @@
 		bts = gsm_bts_by_lac(network, subscr->lac, bts);
 		if (!bts)
 			break;
+
+		/* skip all currently inactive TRX */
+		if (!trx_is_usable(bts->c0))
+			continue;
+
 		num_pages++;
 
 		/* Trigger paging, pass any error to caller */