IB/opa_vnic: Properly clear Mac Table Digest

[ Upstream commit 4bbdfe25600c1909c26747d0b5c39fd0e409bb87 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>