mfd: Fix incorrect kfree(i2c) in wm831x-core i2c_driver probe

The i2c_client received in probe() should not be kfree()'d.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>