usb-host: Drop superfluous null test from usb_host_auto_scan()

Coverity points out that port is later passed to usb_host_open(),
which dereferences it.  It actually can't be null: it always points to
usb_host_scan()'s auto port[].  Drop the superfluous port == NULL
test.

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

hw/usb/host-linux.c

@@ -1760,7 +1760,7 @@ static int usb_host_auto_scan(void *opaque, int bus_num,
         if (f->addr > 0 && f->addr != addr) {
             continue;
         }
-        if (f->port != NULL && (port == NULL || strcmp(f->port, port) != 0)) {
+        if (f->port != NULL && strcmp(f->port, port) != 0) {
             continue;
         }