usb: assert on calling usb_attach(port, NULL) on a port without a dev

with the "usb-ehci: cleanup port reset handling" patch in place no callers
are calling usb_attach(port, NULL) for a port where port->dev is NULL.

Doing that makes no sense as that causes the port detach op to get called
for a port with nothing attached. Add an assert that port->dev != NULL when
dev == NULL, and remove the check for not having a port->dev in the dev == NULL
case.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
