firmware: vpd: use kdtrndup when copying section key

Instead of open-coding kstrndup with kzalloc + memcpy, let's use
the helper.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>