usb: gadget: dummy_hcd: move selection of speed into ->pullup()

The configuration is static however we only know the speed after we have
connected with the other side.

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>