soc: fix comment for freeing soc_dev_attr

The soc_dev_attr is stored soc_dev->attr during soc_device_register() so
it could be used till the cleanup call: soc_device_unregister().
Therefore this memory should not be freed prior, but after unregistering
soc device.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201207185952.261697-1-krzk@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>