HSI: ssi_protocol: Fix return type of ssip_pn_xmit()

With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
indirect call targets are validated against the expected function
pointer prototype to make sure the call target is valid to help mitigate
ROP attacks. If they are not identical, there is a failure at run time,
which manifests as either a kernel panic or thread getting killed. A
proposed warning in clang aims to catch these at compile time, which
reveals:

  drivers/hsi/clients/ssi_protocol.c:1053:20: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .ndo_start_xmit = ssip_pn_xmit,
                            ^~~~~~~~~~~~
  1 error generated.

->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
'netdev_tx_t', not 'int'. Adjust the return type of ssip_pn_xmit() to
match the prototype's to resolve the warning and CFI failure.
Additionally, use the enum 'NETDEV_TX_OK' instead of a raw '0' for the
return value of ssip_pn_xmit().

Link: https://github.com/ClangBuiltLinux/linux/issues/1750Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/hsi/clients/ssi_protocol.c

@@ -968,7 +968,7 @@ static void ssip_xmit_work(struct work_struct *work)
 	ssip_xmit(cl);
 }
 
-static int ssip_pn_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t ssip_pn_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct hsi_client *cl = to_hsi_client(dev->dev.parent);
 	struct ssi_protocol *ssi = hsi_client_drvdata(cl);
@@ -1027,7 +1027,7 @@ static int ssip_pn_xmit(struct sk_buff *skb, struct net_device *dev)
 	dev->stats.tx_packets++;
 	dev->stats.tx_bytes += skb->len;
 
-	return 0;
+	return NETDEV_TX_OK;
 drop2:
 	hsi_free_msg(msg);
 drop:
@@ -1035,7 +1035,7 @@ static int ssip_pn_xmit(struct sk_buff *skb, struct net_device *dev)
 inc_dropped:
 	dev->stats.tx_dropped++;
 
-	return 0;
+	return NETDEV_TX_OK;
 }
 
 /* CMT reset event handler */