USB: make the autosuspend workqueue thread freezable

This patch (as881b) makes the ksuspend_usb_wq workqueue freezable.  We
don't want a rogue workqueue thread running around, unexpectedly
suspending or resuming USB devices in the middle of a system sleep
transition.

This fixes Bugzilla #8498.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>