usb: gadget: move loopback's config descriptor out of f_loopback

f_loopback should only include the bare function but it also includes
the config descriptor. This patch moves the config descriptor into
zero.c, the only user of this function.

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