cosmetic: tighten function type signatures

* change return type to bool
* constify parameters

The nm_is_running(), trx_is_usable() and ts_is_usable() always return
boolean value and are always used as such anyway. The also do not change
their parameters.

Change-Id: I6a572fc78371c69f5308edbad0ebe15e143d0505
diff --git a/src/libbsc/abis_nm.c b/src/libbsc/abis_nm.c
index 295cefd..f1a56a6 100644
--- a/src/libbsc/abis_nm.c
+++ b/src/libbsc/abis_nm.c
@@ -159,7 +159,7 @@
 
 static int abis_nm_rcvmsg_sw(struct msgb *mb);
 
-int nm_is_running(struct gsm_nm_state *s) {
+bool nm_is_running(const struct gsm_nm_state *s) {
 	return (s->operational == NM_OPSTATE_ENABLED) && (
 		(s->availability == NM_AVSTATE_OK) ||
 		(s->availability == 0xff)