USB: usb-skeleton: usb anchor to implement flush

This patch set introduces usb_anchor and uses it to implement all modern
APIs in the skeleton driver.

- proper error reporting in the skeleton driver
- implementation of flush()

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>