[usb-moded] Do not send signals when disconnected and handling -HUP. Fixes: JB#18884

We reread the configuration data after recieving SIGHUP. To make sure this happens
cleanly we set ourselves disconnected. However if we are disconnected this
generates some spurious signals. By using a silent function for this use case
we can avoid it.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>