IB/hfi1: Insure freeze_work work_struct is canceled on shutdown

By code inspection, the freeze_work is never canceled.

Fix by adding a cancel_work_sync in the shutdown path to insure it is no
longer running.

Fixes: 77241056

 ("IB/hfi1: add driver files")
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>