firmware: google: increment VPD key_len properly

Commit 4b708b7b ("firmware: google: check if size is valid when
decoding VPD data") adds length checks, but the new vpd_decode_entry()
function botched the logic -- it adds the key length twice, instead of
adding the key and value lengths separately.

On my local system, this means vpd.c's vpd_section_create_attribs() hits
an error case after the first attribute it parses, since it's no longer
looking at the correct offset. With this patch, I'm back to seeing all
the correct attributes in /sys/firmware/vpd/...

Fixes: 4b708b7b

 ("firmware: google: check if size is valid when decoding VPD data")
Cc: <stable@vger.kernel.org>
Cc: Hung-Te Lin <hungte@chromium.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20190930214522.240680-1-briannorris@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>