i2c: Use IS_REACHABLE() for substituting empty ACPI functions

Replace IS_ENABLED() with IS_REACHABLE() to substitute empty stubs for:
    i2c_acpi_get_i2c_resource()
    i2c_acpi_client_count()
    i2c_acpi_find_bus_speed()
    i2c_acpi_new_device_by_fwnode()
    i2c_adapter *i2c_acpi_find_adapter_by_handle()
    i2c_acpi_waive_d0_probe()

commit f17c06c6 ("i2c: Fix conditional for substituting empty ACPI
functions") partially fixed this conditional to depend on CONFIG_I2C,
but used IS_ENABLED(), which is wrong since CONFIG_I2C is tristate.

CONFIG_ACPI is boolean but let's also change it to use IS_REACHABLE()
to future-proof it against becoming tristate.

Somehow despite testing various combinations of CONFIG_I2C and CONFIG_ACPI
we missed the combination CONFIG_I2C=m, CONFIG_ACPI=y.
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: f17c06c6 ("i2c: Fix conditional for substituting empty ACPI functions")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202408141333.gYnaitcV-lkp@intel.com/Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

include/linux/i2c.h

@@ -1066,7 +1066,7 @@ static inline int of_i2c_get_board_info(struct device *dev,
 struct acpi_resource;
 struct acpi_resource_i2c_serialbus;
 
-#if IS_ENABLED(CONFIG_ACPI) && IS_ENABLED(CONFIG_I2C)
+#if IS_REACHABLE(CONFIG_ACPI) && IS_REACHABLE(CONFIG_I2C)
 bool i2c_acpi_get_i2c_resource(struct acpi_resource *ares,
 			       struct acpi_resource_i2c_serialbus **i2c);
 int i2c_acpi_client_count(struct acpi_device *adev);