HKDF: Difference between salt and info
If all you're using HKDF for is deriving subkeys from a uniformly random master key, you don't really need the Extract part of HKDF.  So you can simply use your master key directly as the IKM input to HKDF-Expand, and the serial number (plus possibly other identifying information, if you e.g. may need to derive multiple keys per card) as the info input.