net: enetc: add missing static descriptor and inline keyword

Fix the build warnings when CONFIG_FSL_ENETC_MDIO is not enabled.
The detailed warnings are shown as follows.

include/linux/fsl/enetc_mdio.h:62:18: warning: no previous prototype for function 'enetc_hw_alloc' [-Wmissing-prototypes]
      62 | struct enetc_hw *enetc_hw_alloc(struct device *dev, void __iomem *port_regs)
         |                  ^
include/linux/fsl/enetc_mdio.h:62:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
      62 | struct enetc_hw *enetc_hw_alloc(struct device *dev, void __iomem *port_regs)
         | ^
         | static
8 warnings generated.

Fixes: 6517798 ("enetc: Make MDIO accessors more generic and export to include/linux/fsl")
Cc: stable@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410102136.jQHZOcS4-lkp@intel.com/
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20241011030103.392362-1-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Wei Fang

include/linux/fsl/enetc_mdio.h

@@ -59,7 +59,8 @@ static inline int enetc_mdio_read_c45(struct mii_bus *bus, int phy_id,
 static inline int enetc_mdio_write_c45(struct mii_bus *bus, int phy_id,
 				       int devad, int regnum, u16 value)
 { return -EINVAL; }
-struct enetc_hw *enetc_hw_alloc(struct device *dev, void __iomem *port_regs)
+static inline struct enetc_hw *enetc_hw_alloc(struct device *dev,
+					      void __iomem *port_regs)
 { return ERR_PTR(-EINVAL); }
 
 #endif