usb: add usb_pick_speed

We can pick the usb port speed in generic code, by looking at the port
and device speed masks and looking for the fastest match.  So add a
function to do exactly that, and drop the speed setting code from
usb_desc_attach as it isn't needed any more.

This way we can set the device speed before calling port->ops->attach,
which fixes some xhci hotplug issues.

https://bugzilla.redhat.com/show_bug.cgi?id=1046873

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
