net: phy: smsc: 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 dedicated helper function.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231012-strncpy-drivers-net-phy-smsc-c-v1-1-00528f7524b3@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>