net: airoha: fix CONFIG_DEBUG_FS check

The #if check causes a build failure when CONFIG_DEBUG_FS is turned
off:

In file included from drivers/net/ethernet/airoha/airoha_eth.c:17:
drivers/net/ethernet/airoha/airoha_eth.h:543:5: error: "CONFIG_DEBUG_FS" is not defined, evaluates to 0 [-Werror=undef]
  543 | #if CONFIG_DEBUG_FS
      |     ^~~~~~~~~~~~~~~

Replace it with the correct #ifdef.

Fixes: 3fe15c6 ("net: airoha: Introduce PPE debugfs support")
Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Lorenzo Bianconi <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: arndb

drivers/net/ethernet/airoha/airoha_eth.h

@@ -540,7 +540,7 @@ void airoha_ppe_deinit(struct airoha_eth *eth);
 struct airoha_foe_entry *airoha_ppe_foe_get_entry(struct airoha_ppe *ppe,
 						  u32 hash);
 
-#if CONFIG_DEBUG_FS
+#ifdef CONFIG_DEBUG_FS
 int airoha_ppe_debugfs_init(struct airoha_ppe *ppe);
 #else
 static inline int airoha_ppe_debugfs_init(struct airoha_ppe *ppe)