ASoC: 88pm860x: refactor deprecated strncpy

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

A suitable replacement is `strscpy` [2] due to the fact that it
guarantees NUL-termination on its destination buffer argument which is
_not_ always the case for `strncpy`!

In this case, though, there was care taken to ensure that the
destination buffer would be NUL-terminated. The destination buffer is
zero-initialized and each `pm860x->name[i]` has a size of
`MAX_NAME_LENGTH + 1`. This means that there is unlikely to be a bug
here.

However, in an attempt to eliminate the usage of the `strncpy` API as
well as disambiguate implementations, replacements such as: `strscpy`,
`strscpy_pad`, `strtomem` and `strtomem_pad` should be preferred.

We are able to eliminate the need for `len + 1` since `strscpy`
guarantees NUL-termination for its destination buffer as per its
implementation [3]:

| /* Hit buffer length without finding a NUL; force NUL-termination. */
| if (res)
| dest[res-1] = '\0';

[1]: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
[2]: manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html
[3]: https://elixir.bootlin.com/linux/v6.3/source/lib/string.c#L183

Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230727-sound-soc-codecs-v1-1-562fa2836bf4@google.com
Signed-off-by: Mark Brown <broonie@kernel.org>