nvmet: avoid unneeded assignment of submit_bio return value

We actually using the cookie returned from the last submit_bio
call.

Signed-off-by: Max Gurtovoy <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Sagi Grimberg <[e-mail]>

drivers/nvme/target/io-cmd.c

···
 			bio_set_op_attrs(bio, op, op_flags);
 
 			bio_chain(bio, prev);
-			cookie = submit_bio(prev);
+			submit_bio(prev);
 		}
 
 		sector += sg->length >> 9;