nvme-loop: remove extra variable in create ctrl

We can call the nvme_change_ctrl_state() directly and have
WARN_ON_ONCE(1) call instead of having to use an extra variable which
matches the name of the function.

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