usb: gadget: update some out of date comments

These functions used to return negative errror codes but now they return
ERR_PTRs.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>