usb: gadget: composite: fix kernel-doc warnings

A few trivial fixes for composite driver:

Warning(include/linux/usb/composite.h:165): No description found for parameter
	'fs_descriptors'
Warning(include/linux/usb/composite.h:165): Excess struct/union/enum/typedef
	member 'descriptors' description in 'usb_function'
Warning(include/linux/usb/composite.h:321): No description found for parameter
	'gadget_driver'
Warning(drivers/usb/gadget/composite.c:1777): Excess function parameter 'bind'
	description in 'usb_composite_probe'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Kosina <trivial@kernel.org>
Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>