The workqueue link_wq should only be destroyed when the hfi1 driver is
unloaded, not when the device is shut down.

Fixes: 71d47008 ("IB/hfi1: Create workqueue for link events")
Link: https://lore.kernel.org/r/20200623204053.107638.70315.stgit@awfm-01.aw.intel.com
Cc: <stable@vger.kernel.org>
Reviewed-by: NMike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: NKaike Wan <kaike.wan@intel.com>
Signed-off-by: NDennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>