cxgbe(4): Use correct synchronization when marking the adapter offline.

adapter->flags are guarded by a synch_op, as noted in the comment in
adapter.h where the flags are defined.

Fixes: 5241b210a4e cxgbe(4): Basic infrastructure for ULDs to participate in adapter reset.
Sponsored by: Chelsio Communications

(cherry picked from commit 52e5a66eac22d24e29f204c5b65232378e71f4cf)