Fix: Add height to Firewall

[StoffelCPR]

Hey,

I've found one of the Firepowers I use has a U_Height of 0.

Unless I'm mistaken this should be 1. Fixed it in this merge request

[harryajc]

Hi @StoffelCPR
I believe this is currently has a U height of 0 so that it fits in a rack mount kit, as it is a small device some of them have been modeled this way if there is a rack mount kit that supports 2 devices in a 1U chassis.