usb: dwc3: gadget: fix stream enable bit

ep->max_streams is a mere hint to the gadget
driver that 'ep' supports stream handling. Using
that as a decision variable for enabling streams
was my worst brain-fart to date.

Instead, we should check from the Superspeed
Endpoint Companion Descriptor if the endpoint
has requested streams. For that we need a little
re-factoring but it is now correct.

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