i2c: Fix conditional for substituting empty ACPI functions

Add IS_ENABLED(CONFIG_I2C) to the conditional around a bunch of ACPI
functions.

The conditional around these functions depended only on CONFIG_ACPI.
But the functions are implemented in I2C core, so are only present if
CONFIG_I2C is enabled.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>