rtos/mqx: prevent crash with -rtos auto

rtos/mqx: prevent crash with -rtos auto

Since mqx comes last in the list, with the auto option its
update_threads is called even though it wasn't detected.

This check should be removed from all the rtos helpers and moved to
the generic code, but better do it later all in one go.

Change-Id: If24ab42a58a468d90e9f12028d4c2fb76a9bc2e8
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2741
Tested-by: jenkins