staging: olpc_dcon: Do not call platform_device_unregister() in dcon_probe()

In dcon_probe(), when platform_device_add() failes to add the device,
it jumps to call platform_device_unregister() to remove the device,
which is unnecessary. So use platform_device_put() instead.

Fixes: 53c43c5c

 ("Revert "Staging: olpc_dcon: Remove obsolete driver"")
Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Link: https://lore.kernel.org/r/20201120074932.31871-1-jingxiangfeng@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>