IB/opa_vnic: Properly clear Mac Table Digest

Clear the MAC table digest when the MAC table is freed.

Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Scott Franco <safranco@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>