gprs_ns2: use gprs_ns2_free_bind() to clean up a bind

gprs_ns2_free_bind() takes care of all required steps to clean up a bind.
The driver->free_bind() operation only cleans up the driver internal state
but not NSVCs and other generic things.
Fixes a crash when free'ing a bind from the vty which has active NSVCs.

Related: OS#5195
Change-Id: I0a2ad22905bcacb929b9b5f5b034af0da3081826
diff --git a/src/gb/gprs_ns2_vty.c b/src/gb/gprs_ns2_vty.c
index 705b60e..2f540c2 100644
--- a/src/gb/gprs_ns2_vty.c
+++ b/src/gb/gprs_ns2_vty.c
@@ -423,7 +423,7 @@
 	vty_bind_free(vbind);
 	bind = gprs_ns2_bind_by_name(vty_nsi, name);
 	if (bind)
-		bind->driver->free_bind(bind);
+		gprs_ns2_free_bind(bind);
 	return CMD_SUCCESS;
 }
 
@@ -678,7 +678,7 @@
 		return CMD_ERR_NOTHING_TODO;
 
 	OSMO_ASSERT(bind->ll == GPRS_NS2_LL_UDP);
-	bind->driver->free_bind(bind);
+	gprs_ns2_free_bind(bind);
 	return CMD_SUCCESS;
 }
 
@@ -939,7 +939,7 @@
 		return CMD_WARNING;
 	}
 
-	bind->driver->free_bind(bind);
+	gprs_ns2_free_bind(bind);
 	return CMD_SUCCESS;
 }