hwmon: (pmbus/core) Use the new i2c_client debugfs directory

The I2C core now manages a debugfs directory per I2C client. PMBus has
its own debugfs hierarchy. Link the two, so a user will be pointed to
the I2C domain from the PMBus domain.

Suggested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>