pm8001: clean bitmap management functions

In the driver two different functions are used to free the same resource,
this patch makes the code easier to read. In addittion to that, some
minor optimisations were made too.

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