[PPPOX]: Fix assignment into const proto_ops.

And actually, with this, the whole pppox layer can basically
be removed and subsumed into pppoe.c, no other pppox sub-protocol
implementation exists and we've had this thing for at least 4
years.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_pppox.h

@@ -157,8 +157,7 @@ struct pppox_proto {
 extern int register_pppox_proto(int proto_num, struct pppox_proto *pp);
 extern void unregister_pppox_proto(int proto_num);
 extern void pppox_unbind_sock(struct sock *sk);/* delete ppp-channel binding */
-extern int pppox_channel_ioctl(struct ppp_channel *pc, unsigned int cmd,
-			       unsigned long arg);
+extern int pppox_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg);
 
 /* PPPoX socket states */
 enum {