usb: gadget: dummy_hcd: move ep initialisation HW setup

This is only required to be done once. There is no counter part to this
in ->stop() so there is no need to re-do it next time. While here also
init the max_stream size to 0 on SS speed.

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>