[mii] Fix typo in parameter name

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/mii.h

  * Initialise MII device
  *
  * @v mii		MII device
- * @v mii		MII interface
+ * @v mdio		MII interface
  * @v address		PHY address
  */
 static inline __attribute__ (( always_inline )) void