sony-laptop: use platform_device_unregister in sony_pf_remove

platform_device_unregister calls platform_device_del and platform_device_put,
thus this change is logically equivalent to original code.

I made this change because the documents in platform.c shows that:
platform_device_del and platform_device_put must _only_ be externally called
in error cases.  All other usage is a bug.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>