greybus: loopback: masked out threads should sleep

If a thread is masked out it should not consume CPU cycles during a test.
We set an arbitrary 100 millisecond sleep time for each masked out thread.
Reasonably blunt instrument to ensure threads with nothing to do don't end
up thrashing the acquisition/release of mutexes.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>