memory corrupted after synce4l_cleanup(cfg)

Hi,

When verifying synce4l on my device, I enter ctrl^C to stop synce4l, I tried to put 2 lines of pr_info in the main function of synce4l.c

```
out:
pr_info("%s:%d end", __func__, __LINE__);
synce4l_cleanup(cfg);
pr_info("%s:%d end", __func__, __LINE__);
pr_info("%s:%d end", __func__, __LINE__);
```

and this is the results:

```
synce4l[169426.640]: [synce4l] QL=0x2 received on Ethernet8
^Csynce4l[169427.427]: [synce4l] synce_clock_destroy
synce4l[169427.427]: [synce4l] trying to detach EEC RCLK for Ethernet8
synce4l[169427.428]: [synce4l] using recover_clock_disable_cmd: echo 0 0 > /sys/class/net/Ethernet8/device/phy/synce on Ethernet8
synce4l[169427.428]: [synce4l] Reply-Str: (null)

synce4l[169427.429]: [synce4l] synce_port_ctrl_destroy on Ethernet8
synce4l[169427.612]: [synce4l] tx thread exit state 3=OK port Ethernet8
synce4l[169427.640]: [synce4l] rx thread exit state 3=OK port Ethernet8
synce4l[169427.700]: [synce4l] main:177 end
synce4l[169427.700]: main:179 end
synce4l[169427.700]: 󒭪ªª main:180 end
```

The message_tag `[synce4l]` has gone at line 179 & 180, and seems the memory is corrupted with `󒭪ªª`