usb: gadget: remove string override from struct usb_composite_driver

The struct usb_composite_driver members iProduct, iSerial and
iManufacturer can be entered directly via the string array. There is no
need for them to appear here.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>