Rod of hellish rebuke interaction with Wall of Fire

Surge of Brimstone Whenever you cast a spell that deals fire or necrotic damage, you can use the rod to deal the maximum damage instead of rolling. Once this property is used, it can’t be used again until the next dawn.

Wall of Fire When the wall appears, each creature in its area makes a Dexterity saving throw, taking 5d8 Fire damage on a failed save or half as much damage on a successful one. One side of the wall, selected by you when you cast this spell, deals 5d8 Fire damage to each creature that ends its turn within 10 feet of that side or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there. The other side of the wall deals no damage.

Does Surge of Brimstone make the Wall of Fire max damage as long as I maintain concentration?

I would say no, as you make a separate damage roll for each time the wall does damage.

However, it's ambiguous, and so ultimately DM's call.

I’d let it work on the initial casting, the “when the wall appears”  based on the “whenever you cast a spell” part. After that, you aren’t casting the spell, you’re concentrating to maintain it. 
I realize this is splitting hairs and maybe overthinking it. So, I’d definitely agree with jl8e that this is a DM ruling situation.

Quote from Xalthu >>

I’d let it work on the initial casting, the “when the wall appears”  based on the “whenever you cast a spell” part. After that, you aren’t casting the spell, you’re concentrating to maintain it. 
I realize this is splitting hairs and maybe overthinking it. So, I’d definitely agree with jl8e that this is a DM ruling situation.

I agree with this interpretation. Some features like this state "when you cast a spell...", which work only during the initial casting of the spell. Other features that work over multiple rounds state something like "when a spell you cast deals damage..." A prime example of this type is the Celestial Warlock's Radiant Soul feature.

Level 6: Radiant Soul

Your link to your patron allows you to serve as a conduit for radiant energy. You have Resistance to Radiant damage. Once per turn, when a spell you cast deals Radiant or Fire damage, you can add your Charisma modifier to that spell’s damage against one of the spell’s targets.