usb: gadget: dummy_hcd: initialize max_streams early

While playing with uasp I noticed that it does not work with dummy_hcd.
The problem is that uasp requires a stream capable endpoint which it is
requesting at bind time like every other gadget. dummy_hcd however
initializes the max_stream value after connect once it knows if it runs
at SS or not.
I don't think that it is might be wrong to initialize the stream
capability even at HS speed. The gadget may not use this descriptor at
HS speed so it should not cause any damage.

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