usb: gadget: composite: add usb_remove_function()

This will be used to remove a single function from a given config. Right
now "ignore" that an error at ->bind() time and cleanup later during
composite_unbind() / remove_config().

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