qlcnic: 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 fw_info->fw_file_name to be NUL-terminated based on its use
within _request_firmware_prepare() wherein `name` refers to it:
|       if (firmware_request_builtin_buf(firmware, name, dbuf, size)) {
|               dev_dbg(device, "using built-in %s\n", name);
|               return 0; /* assigned */
|       }
... and with firmware_request_builtin() also via `name`:
|       if (strcmp(name, b_fw->name) == 0) {

There is no evidence that NUL-padding is required.

Additionally replace size macro (QLC_FW_FILE_NAME_LEN) with
sizeof(fw_info->fw_file_name) to more directly tie the maximum buffer
size to 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>
Signed-off-by: David S. Miller <davem@davemloft.net>