at: reduce delay between retries

There's no reason we should wait 3s before retrying a failed AT command. 
This can even cause issues with suspend inhibition: 3 retries of a 
failed command would take 9s, while logind ignores the inhibitor after 
only 5s.

Reducing the delay to 500ms should be enough and wouldn't interfere with 
suspend inhibition.

Fixes #3

src/at.c

@@ -121,7 +121,7 @@ static void retry_at_command(struct EG25Manager *manager)
         g_critical("Command %s retried %d times, aborting...", at_cmd->cmd, at_cmd->retries);
         next_at_command(manager);
     } else {
-        g_timeout_add_seconds(3, G_SOURCE_FUNC(send_at_command), manager);
+        g_timeout_add(500, G_SOURCE_FUNC(send_at_command), manager);
     }
 }