USB: dummy-hcd: disable interrupts during req->complete

This patch (as756) fixes a bug in dummy-hcd found by the lockdep
checker.  In one of the code paths, the driver did not disable
interrupts before calling a request completion routine.

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