nvmet: remove an unnecessary function parameter to nvmet_check_ctrl_status

In nvmet_check_ctrl_status() cmd can be derived from nvmet_req. Remove
the local variable cmd in the nvmet_check_ctrl_status() and function
parameter cmd for nvmet_check_ctrl_status(). Derive the cmd value from
req parameter in the nvmet_check_ctrl_status().

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>