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: 6517798d ("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.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>