mtd: bcm63xxpart: merge sparelen calculation

The length of the spare part is calculated the same way in both branches
so move to a common place.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>