systemd/serial-log@: Do not fail, if serial port is not working

Currently, the service shows up as failed on systems where the serial
console is not working. As this is expected on several of our systems,
do not list the service unit as failed.

```
$ ls -l /dev/ttyS1
crw-rw---- 1 root dialout 4, 65 Feb 21 13:10 /dev/ttyS1
$ sudo /usr/bin/stty -F /dev/ttyS1 115200 -echo
/usr/bin/stty: /dev/ttyS1: Input/output error
```

misc_systemd_units/serial-log@.service

@@ -2,8 +2,7 @@
 Description=TTY logger
 
 [Service]
-ExecStartPre=/usr/bin/stty -F /dev/%I 115200 -echo
-ExecStart=-/usr/bin/sh -c "/usr/bin/cat /dev/%I >> /var/log/%I.log 2>&1"
+ExecStart=-/usr/bin/sh -c "/usr/bin/stty -F /dev/%I 115200 -echo && /usr/bin/cat /dev/%I >> /var/log/%I.log 2>&1"
 Type=idle
 Restart=always
 RestartSec=0