usb-host: Initialize dev->port the obviously safe way

Coverity worries the strcpy() could overrun the destination.  It
can't, because the source always points to usb_host_scan()'s auto
port[], which has the same size.  Use pstrcpy() anyway, to hush the
checker.

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