[POWERPC] Fix of_i2c include for module compilation

Remove #ifdef CONFIG_OF_I2C as this breaks module compilation.
Drivers using this header should depend on OF_I2C anyways, so
there's no need to make this conditional.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>