ASoC: fsl_micfil: 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 great care taken to ensure that the
destination buffer would be NUL-terminated through the use of `len - 1`
ensuring that the previously zero-initialized buffer would not overwrite
the last NUL byte. This means that there's no bug here.

However, `strscpy` will add a mandatory NUL byte to the destination
buffer as promised by the following `strscpy` implementation [3]:
|       /* Hit buffer length without finding a NUL; force NUL-termination. */
|       if (res)
|               dest[res-1] = '\0';

This means we can lose the `- 1` which clears up whats happening here.
All the while, we get one step closer to eliminating the ambiguous
`strncpy` api in favor of its less ambiguous replacement like `strscpy`,
`strscpy_pad`, `strtomem` and `strtomem_pad` amongst others.

[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>
Acked-by: Shengjiu Wang <shengjiu.wang@gmail.com>
Link: https://lore.kernel.org/r/20230727-sound-soc-fsl-v1-1-4fc0ed7e0366@google.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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