UrfDaemon: fix BlockIdx and Block DBus logic

Both methods incorrectly used priv->invocation when
calling dbus_method_invocation_return_value, when
the parameter invocation should've been used instead.
Also BlockIdx failed to set priv->invocation and
priv->block.  These problems caused urfkill to fail
when generating DBus method returns, causing a timeout
to occur at the caller, even though the underlying
operation succeeded.