USB: gadgetfs cleanups

Minor gadgetfs cleanups:

 - EP0 state constants become consistently STATE_DEV_* rather than
   sometimes omitting the "DEV_"; STATE_EP_* were already consistent.

 - Comment that ep0 state is protected by the spinlock, and update
   code that was neglecting that rule.

None of this is expected to change behavior.

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