pm8001: Fix erratic calculation in update_flash

The loopcount is calculated by using some weird magic. Use instead a boring
macro.

Signed-off-by: Tomas Henzl <thenzl@redhat.com>
Acked-by: Suresh Thiagarajan <Suresh.Thiagarajan@pmcs.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>