"Work ethos" is a compound expression built on the greek word "ethos" (referring to an idea of "habit" that often is ascribed to one's personal/moral tendencies).
With the adjective "work" preceding ethos, the expression refers to a person's work ethic (we can see how "ethic" comes from "ethos" visually in the spelling, too).
"Work ethos" is best understood by the more common (and synonymous) expression, "work ethic."