ifupdown2 defaults for link attributes are not applied

ifupdown2 defaults for link attributes are not applied

Ticket: CM-11718
Reviewed By: CCR-4931
Testing Done: Tested complete regression suite on hardnode in 3.0.

This patch fixes a problem in the ethtool addon module where a single iface stanza was
configured for a link-speed (1G) other then the default (10G). The link-speed config is then
removed or commented out but the link-speed is not restored to its default value (10G) because of
incorrect logic.