greybus: Add workqueue to handle vibrator timeout

In the beginning, module side can control the vibrator timeout value,
it can disable vibrator until timeout. But after Runtime PM control added
in, AP side didn't know when module can be suspended, the vibrator
task will be interrupted by suspending event.

Because of this problem, the module can not be in charge of counting down
the timeout value, it is now up to the AP to manage this. So add workqueue
to handle the vibrator timeout.

Signed-off-by: Ann Chen <chen_ann@projectara.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>