i2c: demux: handle the new atomic callbacks

If the parent has an atomic callback, we need to translate it the same
way as the non-atomic callback.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>