net: phy: fix sparse warning in fixed.c

This commit fixes the following sparse warning:

drivers/net/phy/fixed.c:207
    - warning: symbol 'fixed_phy_del' was not declared.
      Should it be static?

by adding symbol definition to the phy_fixed.h API file. It is ok to do
because the function in question is an exported symbol.
Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/phy_fixed.h

@@ -17,6 +17,7 @@ extern int fixed_phy_add(unsigned int irq, int phy_id,
 extern int fixed_phy_register(unsigned int irq,
 			      struct fixed_phy_status *status,
 			      struct device_node *np);
+extern void fixed_phy_del(int phy_addr);
 #else
 static inline int fixed_phy_add(unsigned int irq, int phy_id,
 				struct fixed_phy_status *status)
@@ -29,6 +30,10 @@ static inline int fixed_phy_register(unsigned int irq,
 {
 	return -ENODEV;
 }
+static inline int fixed_phy_del(int phy_addr)
+{
+	return -ENODEV;
+}
 #endif /* CONFIG_FIXED_PHY */
 
 /*