mgcp_osmux: available circuit IDs from 0 to 255, not from 0 to 128

Holger reports that the bitmap that accounts for available Osmux circuit
IDs is limited to 128, when the maximum number of circuit IDs are
determined by the uint8_t field in the header (ie. 256 circuits).

[hfreyther: Update the testcase now that we have more ids to allocate]
diff --git a/openbsc/tests/mgcp/mgcp_test.c b/openbsc/tests/mgcp/mgcp_test.c
index ec86c55..6c94002 100644
--- a/openbsc/tests/mgcp/mgcp_test.c
+++ b/openbsc/tests/mgcp/mgcp_test.c
@@ -1186,7 +1186,7 @@
 	osmux_put_cid(id);
 	OSMO_ASSERT(osmux_used_cid() == 0);
 
-	for (i = 0; i < 128; ++i) {
+	for (i = 0; i < 256; ++i) {
 		id = osmux_get_cid();
 		OSMO_ASSERT(id == i);
 		OSMO_ASSERT(osmux_used_cid() == i + 1);