dpll: fix build failure due to rcu_dereference_check() on unknown type

Tasmiya reports that their compiler complains that we deref
a pointer to unknown type with rcu_dereference_rtnl():

include/linux/rcupdate.h:439:9: error: dereferencing pointer to incomplete type ‘struct dpll_pin’

Unclear what compiler it is, at the moment, and we can't report
but since DPLL can't be a module - move the code from the header
into the source file.

Fixes: 0d60d8df ("dpll: rely on rcu for netdev_dpll_pin()")
Reported-by: Tasmiya Nalatwad <tasmiya@linux.vnet.ibm.com>
Link: https://lore.kernel.org/all/3fcf3a2c-1c1b-42c1-bacb-78fdcd700389@linux.vnet.ibm.com/Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240229190515.2740221-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/dpll/dpll_core.c

@@ -42,6 +42,11 @@ struct dpll_pin_registration {
 	void *priv;
 };
 
+struct dpll_pin *netdev_dpll_pin(const struct net_device *dev)
+{
+	return rcu_dereference_rtnl(dev->dpll_pin);
+}
+
 struct dpll_device *dpll_device_get_by_id(int id)
 {
 	if (xa_get_mark(&dpll_device_xa, id, DPLL_REGISTERED))

include/linux/dpll.h

@@ -169,13 +169,13 @@ int dpll_device_change_ntf(struct dpll_device *dpll);
 
 int dpll_pin_change_ntf(struct dpll_pin *pin);
 
+#if !IS_ENABLED(CONFIG_DPLL)
 static inline struct dpll_pin *netdev_dpll_pin(const struct net_device *dev)
 {
-#if IS_ENABLED(CONFIG_DPLL)
-	return rcu_dereference_rtnl(dev->dpll_pin);
-#else
 	return NULL;
-#endif
 }
+#else
+struct dpll_pin *netdev_dpll_pin(const struct net_device *dev);
+#endif
 
 #endif