[PATCH net-next v3] hdlc_ppp: fix potential null pointer in ppp_cp_event logging

[Kriish Sharma <kriish.sharma2006@gmail.com>]

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 <khalasa@piap.pl>
Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
---
v3:
  - Retarget patch to net-next (cleanup, not bugfix)
  - Remove 'Fixes' tag
  - Drop explicit PID_LCP case in proto_name() per Chris’s suggestion

v2: https://lore.kernel.org/all/20251003092918.1428164-1-kriish.sharma2006@gmail.com/
  - Target the net tree with proper subject prefix "[PATCH net]"
  - Update proto_name() to return "LCP" by default instead of NULL
  - Remove local 'pname' variable in ppp_cp_event
  - Add Suggested-by tag for Krzysztof Hałasa

v1: https://lore.kernel.org/all/20251002180541.1375151-1-kriish.sharma2006@gmail.com/

 drivers/net/wan/hdlc_ppp.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/net/wan/hdlc_ppp.c b/drivers/net/wan/hdlc_ppp.c
index 7496a2e9a282..159295c4bd6d 100644
--- a/drivers/net/wan/hdlc_ppp.c
+++ b/drivers/net/wan/hdlc_ppp.c
@@ -126,14 +126,12 @@ static inline struct proto *get_proto(struct net_device *dev, u16 pid)
 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";
 	}
 }
 
-- 
2.34.1