'Salt Pork' can refer to a few different cuts of preserved pork, but it is always uncured/unsmoked. So, salt is the only preservative, and as such it is quite salty. There are different ways of cooking it, but the first step is typically boiling it or soaking it to make it less salty. 'To soak', means 'to make saturated with water,' and in your sentence, 'soaking for dinner' just means it is currently immersed in water because it is going to be cooked for dinner.
The two phrases are mostly interchangeable. If you were to use "the soaking salt pork" then you should leave off the 'for dinner' at the end because it sounds a little weird to have it far away from the verb it is modifying (soaking).