usb: gadget: configfs: log function unbinding as debug

Disabling USB gadget functions configured through configfs is something
that can happen in normal use cases. Keep the existing log for this type
of event, but only as debug, not as an error.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>