USB: fix skeleton driver

compilation of the skeleton driver is currently broken. It doesn't compile.
So while I am it:

- fix typo
- add comments to answer common questions
- actually allow autosuspend in the driver struct
- increase paralellism by restricting code under locks

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