wago: Fix setter on fs10 used as vectors

Closes #3787 (closed)

This PR fixes a problem when a "fs" module is used as a vector (same name for multiple physical modules).

Plus a test on that use case

-                    offset, "H" * size, val, timeout=self.timeout
+                    offset, "H" * len(val), val, timeout=self.timeout

I think it's safe, as seen with @matias.guijarro, because the number of "H" anyway have to be consistent with the number of elements of the val.