regulator: palmas: Fix palmas_probe error handling

Fix below error handling cases:

1. If reading PALMAS_SMPS_CTRL fails, simply returns ret rather than goto
   err_unregister_regulator because we have not call regulator_register().
2. If palmas_ldo_init() fails, we need to call regulator_unregister() for the
   regulator we just successfully registered in this for loop iteration.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>