Put a levels adjustment layer above the text layer. Hold down the alt key and hover your mouse at the line between the two layers, the pointer will change (what it looks like exactly depends on which version of photoshop you use) and then click. The adjustment layer will have a new little arrow pointing down to the text layer. The adjustment layer is now clipped to the text layer below it, so any adjustments you make will only be applied to the text layer below, and not anything else underneath.
Control which layers are affected by an adjustment layer | Adobe Photoshop CC tutorials
Mike