The length of the spare part is calculated the same way in both branches
so move to a common place.
Signed-off-by: NJonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: NFlorian Fainelli <florian@openwrt.org>
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>