NVMe: Correct uses of INIT_WORK

We need to initialise the work_struct when we initialise the rest of the
struct nvme_dev, otherwise we'll hit a lockdep warning when we remove
the device.  Use PREPARE_WORK to change the function pointer instead
of INIT_WORK.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>