i2c: Make i2c_register_board_info() a NOP when CONFIG_I2C_BOARDINFO=n

Don't require platform code to be #ifdeffed according to whether
I2C is enabled or not ... if it's not enabled, let GCC compile out
all I2C device declarations.  (Issue noted on an NSLU2 build that
didn't configure I2C.)
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

include/linux/i2c.h

@@ -271,9 +271,16 @@ extern void i2c_unregister_device(struct i2c_client *);
  * This is done at arch_initcall time, before declaring any i2c adapters.
  * Modules for add-on boards must use other calls.
  */
+#ifdef CONFIG_I2C_BOARDINFO
 extern int
 i2c_register_board_info(int busnum, struct i2c_board_info const *info, unsigned n);
-
+#else
+static inline int
+i2c_register_board_info(int busnum, struct i2c_board_info const *info, unsigned n)
+{
+	return 0;
+}
+#endif
 
 /*
  * The following structs are for those who like to implement new bus drivers: