nvme: fix directive command numd calculation

The numd field of directive receive command takes number of dwords to
transfer. This fix has the correct calculation for numd.

Signed-off-by: Kwan (Hingkwan) Huen-SSI <kwan.huen@samsung.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christoph Hellwig <hch@lst.de>