wifi: ath5k: replace deprecated strncpy with strscpy

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

We expect led->name to be NUL-terminated based on the presence of a
manual NUL-byte assignment.

This NUL-byte assignment was added in Commit daf9669b ("ath5k:
ensure led name is null terminated"). If strscpy() had existed and had
been used back when this code was written then potential bugs and the
need to manually NUL-terminate could have been avoided. Since we now
have the technology, let's use it :)

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding. If NUL-padding is required let's opt
for strscpy_pad().

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


Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231013-strncpy-drivers-net-wireless-ath-ath5k-led-c-v1-1-3acb0b5a21f2@google.com