nvmet: remove redundant assignment after left shift

The left shift is followed by a re-assignment back to cc_css, the
assignment is redundant.  Fix this by replacing the "<<=" operator with
"<<" instead.

This cleans up the clang scan build warning:

drivers/nvme/target/core.c:1124:10: warning: Although the value stored to 'cc_css' is used in the enclosing expression, the value is never actually read from 'cc_css' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>