nvme: Move opcode string helper functions declarations

nvme: Move opcode string helper functions declarations

Move the declaration of all helper functions converting NVMe command
opcodes and status codes into strings from drivers/nvme/host/nvme.h
into include/linux/nvme.h, together with the commands definitions.
This allows NVMe target drivers to call these functions without having
to include a host header file.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Rick Wertenbroek <rick.wertenbroek@gmail.com>
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>