USB: gadget: dummy_hcd: implement set_wedge

This patch (as1131) implements the set_wedge() method for dummy_hcd.
This method is necessary for strict USBCV compliance in
g_file_storage.

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