i2c: powermac: use true,false for bool variable

In i2c_powermac_register_devices(), variable 'found_onyx' is bool and
assigned '0' and 'true' in different places. Use 'false' instead of '0'.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>