icera: fix parsing of IP config options

Some of the IP address items will be 0.0.0.0 depending on what the
other items are, like when the duplicate gateway is set on newer
devices, the first gateway address may be 0.0.0.0.  Since that's
not a valid IP address, just don't set that member of the config.

Second, the intent with the duplicate gateway is only to use that
when the first gateway was not given (ie, was 0.0.0.0) so fix the
check for that.