usb: dwc3: gadget: implement streams support

The following patch adds support for streams
to dwc3 driver.

While at that, also fix one small issue on
endpoint disable where we should clear all
flags not only ENABLED.

Reviewied-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>