osmux: fix show online-help

Before:

    <command id='osmux dummy (on|off)'>
      <params>
        <param name='osmux' doc='RTP multiplexing' />
        <param name='dummy' doc='Enable dummy padding' />
        <param name='on' doc='Disable dummy padding' />
        <param name='off' doc='(null)' />
      </params>

After:

    <command id='osmux dummy (on|off)'>
      <params>
        <param name='osmux' doc='RTP multiplexing' />
        <param name='dummy' doc='Dummy padding' />
        <param name='on' doc='Enable dummy padding' />
        <param name='off' doc='Disable dummy padding' />
      </params>
    </command>

Note the 'null' string in 'off'. Reported by Holger.
diff --git a/openbsc/src/libmgcp/mgcp_vty.c b/openbsc/src/libmgcp/mgcp_vty.c
index a620a20..fc2f818 100644
--- a/openbsc/src/libmgcp/mgcp_vty.c
+++ b/openbsc/src/libmgcp/mgcp_vty.c
@@ -1292,7 +1292,7 @@
 DEFUN(cfg_mgcp_osmux_dummy,
       cfg_mgcp_osmux_dummy_cmd,
       "osmux dummy (on|off)",
-      OSMUX_STR "Enable dummy padding\n" "Disable dummy padding\n")
+      OSMUX_STR "Dummy padding\n" "Enable dummy padding\n" "Disable dummy padding\n")
 {
 	if (strcmp(argv[0], "on") == 0)
 		g_cfg->osmux_dummy = 1;