RDMA/i40iw: Do not set self-referencing pointer to NULL after free

iwqp->allocated_buffer is a self-referencing pointer to iwqp.
Do not set iwqp->allocated_buffer to NULL after freeing it.

Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>