nvme: move ->retries setup to nvme_setup_cmd

->retries is counting the number of times a command is resubmitted, and
be cleared on the first time we see the command.  We currently don't do
that for non-PCIe command, which is easily fixed by moving the setup
to common code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>