mfd: Fix tps6586x_add_subdevs error path

1. return -ENOMEM if platform_device_alloc() fail.
2. call platform_device_put() if platform_device_add() fail.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>