settings: Fix uninitialised variables

When compiling the settings subsystems, I was getting the following
compiler warning:

```
/home/markus/src/wrp-n4m/zephyr/subsys/settings/src/settings_line.c: In function 'settings_line_cmp':
/home/markus/src/wrp-n4m/zephyr/subsys/settings/src/settings_line.c:477:6: warning: 'rc' may be used uninitialized in this function [-Wmaybe-uninitialized]
  477 |  int rc;
      |      ^~
/home/markus/src/wrp-n4m/zephyr/subsys/settings/src/settings_line.c: In function 'settings_line_entry_copy':
/home/markus/src/wrp-n4m/zephyr/subsys/settings/src/settings_line.c:453:9: warning: 'rc' may be used uninitialized in this function [-Wmaybe-uninitialized]
  453 |  return rc;
```

This patch fixes the warning.

Signed-off-by: Markus Becker <markus.becker@tridonic.com>