IB/qib: Move COUNTER_MASK definition within qib_mad.h header guards

Commit 36a8f01c ("IB/qib: Add congestion control agent
implementation") caused statements to leak pass the header guard.
Fix this.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>