nvme-pci: remove redundant dma frees in hmb

The value of size is 0 when there is no dma buffer allocated. The value
of i also remains 0. So, no need to free the dma buffer in out_free_bufs.
Hence, remove the redundant dma frees.

Signed-off-by: Francis Pravin <francis.p@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>