USB: "sparse" cleanups for usb gadgets

This removes complaints about the gadget stack which are generated by
the currrent "sparse":  it doesn't like the fact that zero is the null
pointer.  (Last I checked, C guarantees that's correct ...)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>