The suggested way to add a race to NPC is to simply add the racial features from the NPC Stat Blocks section of the Dungeon Masters' Workshop in the DMG, as per the answer to How do you add a race to a race-optional monster?
More specifically to this case, adding the Sverfneblin racial features to the Evil Mage stat block in this way doesn't change the Intelligence Ability score or INT modifier and, as the spellcasting save DC is set as per David Coffron's answer and Step 12 of Creating a Monster Stat Block:
You can determine save DCs in one of two ways.
Use the Table. You can start with the monster’s expected challenge rating and use the Monster Statistics by Challenge Rating table to determine an appropriate save DC for any effect that requires a target to make a saving throw.
Calculate the DCs. Alternatively, you can calculate a monster’s save DCs as follows: 8 + the monster’s proficiency bonus + the monster’s relevant ability modifier. You choose the ability that best applies.
There is nothing in the DMG (that I can see) that states a DC would be affected by adding Innate Spellcasting, so in this case, with no relevant ability changes, it remains the same as the base monster.