bsc_vty: Fix unknown 'use-dtx' config parser error

commit 5a3a61d88f1231af5dba96050cbc27270765ef23 introduces a DTX attribute
which is parsed as dtx-used, but written as use-dtx (and thus causes a parser
error).
diff --git a/openbsc/src/bsc_vty.c b/openbsc/src/bsc_vty.c
index b784465..fe8d5be 100644
--- a/openbsc/src/bsc_vty.c
+++ b/openbsc/src/bsc_vty.c
@@ -547,7 +547,7 @@
 	vty_out(vty, " timer t3117 %u%s", gsmnet->T3117, VTY_NEWLINE);
 	vty_out(vty, " timer t3119 %u%s", gsmnet->T3119, VTY_NEWLINE);
 	vty_out(vty, " timer t3141 %u%s", gsmnet->T3141, VTY_NEWLINE);
-	vty_out(vty, " use-dtx %u%s", gsmnet->dtx_enabled, VTY_NEWLINE);
+	vty_out(vty, " dtx-used %u%s", gsmnet->dtx_enabled, VTY_NEWLINE);
 
 	return CMD_SUCCESS;
 }