NVMe: Correct the definitions of two ioctls

NVME_IOCTL_SUBMIT_IO has a struct nvme_user_io, not a struct nvme_rw_command
as a parameter, and NVME_IOCTL_DOWNLOAD_FW is a Write, not a Read.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>