wifi: wlcore: boot: 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 wl->chip.fw_ver_str to be NUL-terminated based on its usage
with DRIVER_STATE_PRINT_STR() in debugfs.c:
491 | DRIVER_STATE_PRINT_STR(chip.fw_ver_str);
... which uses DRIVER_STATE_PRINT():
444 | #define DRIVER_STATE_PRINT_STR(x)  DRIVER_STATE_PRINT(x, "%s")
... which relies on scnprintf:
434 | #define DRIVER_STATE_PRINT(x, fmt)   \
435 | 	(res += scnprintf(buf + res, DRIVER_STATE_BUF_LEN - res,\
436 | 			  #x " = " fmt "\n", wl->x))

Moreover, NUL-padding is not required.

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
Similar-to: https://lore.kernel.org/all/20231018-strncpy-drivers-net-wireless-ti-wl18xx-main-c-v2-1-ab828a491ce5@google.com/
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231018-strncpy-drivers-net-wireless-ti-wlcore-boot-c-v1-1-d3c6cc6b80fe@google.com