usb: gadget: configfs: use to_usb_function_instance() in cfg (un)link func

replace open-coded container_of() with to_usb_function_instance() helper.

Reviewed-by: Jack Pham <quic_jackp@quicinc.com>
Signed-off-by: Linyu Yuan <quic_linyyuan@quicinc.com>
Link: https://lore.kernel.org/r/1637211213-16400-5-git-send-email-quic_linyyuan@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>