usb: gadget: move USB_BUFSIZ into global composite.h

This patch moves USB_BUFSIZ into global header file as
USB_COMP_EP0_BUFSIZ. There is currently only one user (f_sourcesink)
besides composite which need it. Ideally f_sourcesink would have its
own ep0 buffer. Lets keep it that way it was for now.

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