usb-host: properly release port on unplug & exit

Factor out port release into a separate function.  Call release function
in exit notifier too.  Add explicit call the USBDEVFS_RELEASE_PORT
ioctl, just closing the hub file handle seems not to be enougth.  Make
sure we release the port before resetting the device, otherwise host
drivers will not re-attach.

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