usb: dwc3: gadget: initialize max_streams

Without this the gadget will never be able to allocate a stream capable
endpoint. The manual says that the stream id is a 16bit id. It does not
talk about an upper limit in any other way. So I think 15 is a
reasonable limit :)

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