pm8001: Update nvmd response data to request buffer

Instead of using the virt_ptr use request buffer for copying
back the nvmd response data and use the same in request function also

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