greybus: interface: Receive serial-number on hotplug event

Two exactly same modules can be uniquely identified using module's
serial-number. This patch updates the interface hotplug event to also
receive the serial-number of the module.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>