udev/net: initialize coalesce tristate variables
Otherwise, 99-default.link may introduce something like the
following warnings:
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Aug 26 03:23:59 systemd-udevd[519]: wlan0: Could not set coalesce settings, ignoring: Operation not supported
Aug 26 03:24:00 systemd-udevd[547]: wlp59s0: Could not set coalesce settings, ignoring: Operation not supported
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Follow-up for 6c35ea5ef0231d519ff24d43a57a72cebab6a121.