pm8001: fix pm8001_store_update_fw

The current implementation may mix the negative value returned from
pm8001_set_nvmd with count. -(-ENOMEM) could be interpreted as bytes
programmed, this patch fixes it.

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