greybus: svc: Handle hotplug request from a workqueue

Bringing up a module can be time consuming, as that may require lots of
initialization on the module side. Over that, we may also need to
download the firmware first and flash that on the module.

In order to make other hotplug events to not wait for all this to
finish, handle most of module hotplug stuff outside of the hotplug
callback, with help of a workqueue.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>