nvme: drop unused variable ctrl in nvme_setup_cmd

The variable 'ctrl' became useless since the code using it was dropped
from nvme_setup_cmd() in the commit 292ddf67bbd5 ("nvme: increment
request genctr on completion"). Fix it to get rid of this compilation
warning in the nvme-5.17 branch:

 drivers/nvme/host/core.c: In function ‘nvme_setup_cmd’:
 drivers/nvme/host/core.c:993:20: warning: unused variable ‘ctrl’ [-Wunused-variable]
   struct nvme_ctrl *ctrl = nvme_req(req)->ctrl;
                     ^~~~

Fixes: 292ddf67bbd5 ("nvme: increment request genctr on completion")
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>