usb: gadget: remove bufsiz from struct usb_composite_dev

there is no read user of bufsiz, its content is available via
USB_COMP_EP0_BUFSIZ. Remove it.

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