isdn: 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 `iclock->name` to be NUL-terminated based on its use within
printk:
|	printk(KERN_DEBUG "%s: %s %d\n", __func__, iclock->name,
|	       iclock->pri);

`iclock` is zero-initialized and as such is already NUL-padded which
means strncpy is doing extra work here by eagerly NUL-padding the
destination buffer.

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.

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>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230922-strncpy-drivers-isdn-misdn-clock-c-v1-1-3ba2a5ae627a@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>