networkmanager: Fix "Ethernet MTU" dialog layout
networkmanager: Fix "Ethernet MTU" dialog layout

The CSS wasn't correctly updated in

    fdb7237 * networkmanager: Rename "ethernet settings" to just "mtu settings"

Closes #5947
Reviewed-by: Dominik Perpeet <f7830b2723a5db4fc185058b06cfab4b89a38da9@redhat.com>

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