ROCK/POP ''Streets of Fire.'' Music from the original motion picture sound track. (MCA Records MCA-5492.) - I'll reveal my fondness for 1950s rock by admitting that my favorite musical moment of ''Streets of Fire, a garish ''Rock & Roll Fable'' directed by Walter Hill, is the sudden appearance of Link Wray's classic ''Rumble'' in the middle of an otherwise modern score. Similarly, my favorite cut on the sound track album is the doo-wop flavored ''Countdown to Love'' by Greg Phillinganes. The rest is standard pop in various current veins by the Blasters and the Fixx, among others. Except for Ry Cooder's smarmy ''Hold That Snake'' the collection is less show-offy than the movie that spawned it, but there's less good humor here, too.