usb: composite.h: fix two warnings when building docs

By definition, we use /* private: */ tag when we won't be documenting
a parameter. However, those two parameters are documented:

./include/linux/usb/composite.h:510: warning: Excess struct/union/enum/typedef member 'setup_pending' description in 'usb_composite_dev'
./include/linux/usb/composite.h:510: warning: Excess struct/union/enum/typedef member 'os_desc_pending' description in 'usb_composite_dev'

So, we need to use /* public: */ to avoid a warning.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>