crypto: cavium/nitrox - refactor deprecated strncpy

crypto: cavium/nitrox - refactor deprecated strncpy

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

We know `hw.partname` is supposed to be NUL-terminated by its later use with seq_printf:
| nitrox_debugfs.c +25
|      seq_printf(s, "  Part Name: %s\n", ndev->hw.partname);

Let's prefer a more robust and less ambiguous string interface.

A suitable replacement is `strscpy` [2] due to the fact that it guarantees
NUL-termination on the destination buffer.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>