QSerialPortInfo FreeBSD: avoid variable shadowing

The availablePorts() implementation has two loops over nodes, where
the second one shadows the outer variable:

  for (const NodeInfo &node : nodes) {
    // ...
    for (const NodeInfo &node : nodes) {
      // ...
    }
  }

Rename the inner variable to descriptionNode, because it's used to
populate the device description and manufacturer information.

Pick-to: 6.12
Change-Id: I5c1f89ac49b118bcb2c78f4c2bc31e4410f89a46
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>