vss: Fix offset handling.

vss: Fix offset handling.

Commit fb6d93240 (vss: Update offset directly when setting the REPOS
flag.) was buggy. On pause/restart we miss to update mmd->offset,
hence the status items which are sent to the stat clients like audiod
did not contain the correct offset value either which messes up
the time string that audiod sends to its clients (para_gui).

We need to set the new offset both on repos requests and on restart,
do introduce a small helper function for this purpose.