i2c: mux: populate the new *_atomic callbacks

If the parent adapter has atomic_xfer callbacks, populate them for the
mux adapter as well. We can use the same translation function as for the
non-atomic xfer callback.

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