Simplify add_mux/del_mux support in qmi_wwan

The original logic assumed that after the sysfs add_mux/del_mux write operation, the link interface would require some time to show up in sysfs, and so we would attempt to always run one by one these operations, so that e.g. we could keep track of what mux_id was requested and what link name was generated.

But this doesn't seem to be true; add_mux/del_mux seem to guarantee that as soon as the write operation finishes successfully, the new link is already available (for add_mux) or the given link was already removed (for del_mux).

So just simplify the logic, and run the whole add/del logic in a sync way without forcing an artificial 250ms timeout to complete each operation.