iommu/vt-d: Fix NULL pointer dereference in prq_event_thread()

When handling page request without pasid event, go to "no_pasid"
branch instead of "bad_req". Otherwise, a NULL pointer deference
will happen there.

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Fixes: a222a7f0 'iommu/vt-d: Implement page request handling'
Signed-off-by: Joerg Roedel <jroedel@suse.de>