firmware: vpd: use kasprintf() when forming name of 'raw' attribute

When creating name for the "raw" attribute, let's switch to using
kaspeintf() instead of doing it by hand. Also make sure we handle
errors.

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>