fix switch will forget the default link speed for unknown

For network devices those failed to retrieve the link status
will have a default value of 100Mbps FD TP at the first scan.
But during the next scan by monitor update, the default value
will be lost.

Signed-off-by: Hiroaki KAWAI <kawai@stratosphere.co.jp>
Merge pull request #34 from iHiroakiKawai/port_link_status_update

fix switch will forget the default link speed for unknown