bnxt_en: Fix NULL pointer dereference in a failure path during open.

If bnxt_hwrm_ring_free() is called during a failure path in bnxt_open(),
it is possible that the completion rings have not been allocated yet.
In that case, the completion doorbell has not been initialized, and
calling bnxt_disable_int() will crash.  Fix it by checking that the
completion ring has been initialized before writing to the completion
ring doorbell.
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -3134,7 +3134,9 @@ static void bnxt_disable_int(struct bnxt *bp)
 	for (i = 0; i < bp->cp_nr_rings; i++) {
 		struct bnxt_napi *bnapi = bp->bnapi[i];
 		struct bnxt_cp_ring_info *cpr = &bnapi->cp_ring;
+		struct bnxt_ring_struct *ring = &cpr->cp_ring_struct;
 
+		if (ring->fw_ring_id != INVALID_HW_RING_ID)
 			BNXT_CP_DB(cpr->cp_doorbell, cpr->cp_raw_cons);
 	}
 }