usb: gadget: epautoconf: add usb_ep_autoconfig_release() function

This patch introduces usb_ep_autoconfig_release() function which allows
to release endpoint previously obtained from usb_ep_autoconfig() during
USB function bind.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>