fastboot: sparse: fix block addressing for don't care chunk type

When 7bfc3b13 (sparse: Refactor chunk parsing function) was implemented,
it dropped 99819458 (aboot: fix block addressing for don't care chunk type).

This re-implements the required fix for the "don't care chunk type"...

Signed-off-by: Steve Rae <srae@broadcom.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>