USB: support for autosuspend in sierra while online

This implements support for autosuspend in the sierra driver while online.
Remote wakeup is used for reception. Transmission is facilitated with a queue
and the asynchronous autopm mechanism. To prevent races a private flag
for opened ports and a counter of running transmissions needs to be added.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Tested-by: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>