dm: usb: Do not reprobe usb hosts on "usb tree" command

Some usb hosts may have failed to probe on "usb start", i.e. an otg
host without an otg-host cable plugged in.

"usb tree" would cause the probe method of these hosts to get called
again, something which should only happen on "usb reset".

This commit fixes this.

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