net: dsa: b53: Fix build with B53_SRAB enabled and not B53_SERDES

In case B53_SRAB is enabled, but not B53_SERDES, we can get the
following linking error:

ERROR: "b53_serdes_init" [drivers/net/dsa/b53/b53_srab.ko] undefined!

We also need to ifdef the body of b53_srab_serdes_map_lane() since it
would not be used when B53_SERDES is disabled and that would produce a
warning.

Fixes: 0e01491d ("net: dsa: b53: Add SerDes support")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/b53/b53_serdes.h

@@ -118,4 +118,11 @@ void b53_serdes_link_set(struct b53_device *dev, int port, unsigned int mode,
 void b53_serdes_phylink_validate(struct b53_device *dev, int port,
 				unsigned long *supported,
 				struct phylink_link_state *state);
+#if IS_ENABLED(CONFIG_B53_SERDES)
 int b53_serdes_init(struct b53_device *dev, int port);
+#else
+static inline int b53_serdes_init(struct b53_device *dev, int port)
+{
+	return -ENODEV;
+}
+#endif

drivers/net/dsa/b53/b53_srab.c

@@ -390,6 +390,7 @@ static irqreturn_t b53_srab_port_isr(int irq, void *dev_id)
 	return IRQ_WAKE_THREAD;
 }
 
+#if IS_ENABLED(CONFIG_B53_SERDES)
 static u8 b53_srab_serdes_map_lane(struct b53_device *dev, int port)
 {
 	struct b53_srab_priv *priv = dev->priv;
@@ -407,6 +408,7 @@ static u8 b53_srab_serdes_map_lane(struct b53_device *dev, int port)
 		return B53_INVALID_LANE;
 	}
 }
+#endif
 
 static int b53_srab_irq_enable(struct b53_device *dev, int port)
 {