net: dsa: mt7530: replace deprecated strncpy with ethtool_sprintf

net: dsa: mt7530: replace deprecated strncpy with ethtool_sprintf

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

ethtool_sprintf() is designed specifically for get_strings() usage.
Let's replace strncpy in favor of this more robust and easier to
understand interface.

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
Signed-off-by: Justin Stitt <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Daniel Golle <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Reviewed-by: AngeloGioacchino Del Regno <[email protected]>
Link: https://lore.kernel.org/r/20231009-strncpy-drivers-net-dsa-mt7530-c-v1-1-ec6677a6436a@google.com
Signed-off-by: Jakub Kicinski <[email protected]>