IB/hfi1: Fix potential NULL ptr dereference

This fixes potential NULL ptr dereference because IS_ERR(dd) doesn't
handle NULL. Fix the issue by initializing the pointer with a not NULL
error code.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>