RDMA/cxgb3: Annotate an RCU pointer

Annotate t3cdev.l2opt with __rcu since it is used as an RCU pointer.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>