hdlc_ppp: fix potential null pointer in ppp_cp_event logging

drivers/net/wan/hdlc_ppp.c: In function ‘ppp_cp_event’:
drivers/net/wan/hdlc_ppp.c:353:17: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
353 | netdev_info(dev, "%s down\n", proto_name(pid));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wan/hdlc_ppp.c:342:17: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
342 | netdev_info(dev, "%s up\n", proto_name(pid));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Update proto_name() to return "LCP" by default instead of NULL.
This change silences the compiler without changing existing behavior
and removes the need for the local 'pname' variable in ppp_cp_event.

Suggested-by: Krzysztof Hałasa <[e-mail]>
Signed-off-by: Kriish Sharma <[e-mail]>
Acked-by: Krzysztof Hałasa <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/wan/hdlc_ppp.c

···
 static inline const char *proto_name(u16 pid)
 {
 	switch (pid) {
-	case PID_LCP:
-		return "LCP";
 	case PID_IPCP:
 		return "IPCP";
 	case PID_IPV6CP:
 		return "IPV6CP";
 	default:
-		return NULL;
+		return "LCP";
 	}
 }