USB: usb.h: change private: kernel-doc for new format requirement

Use "/* private:" to mark struct members as private so that
scripts/kernel-doc will handle them correctly.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>