accessibility: speakup: refactor deprecated strncpy

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

Let's refactor this function to just use synth_write().

Link: 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>
Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20230918-strncpy-drivers-accessibility-speakup-kobjects-c-v2-1-d5b1976c5dbf@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>