firmware: dmi: Use the proper accessor for the version field

The byte at offset 6 represents length. Don't take it and drop it
immediately by using proper accessor, i.e. get_unaligned_be24().

[JD: Change the subject to something less frightening]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jean Delvare <jdelvare@suse.de>