This patch create root directory at hisi_sas_init() and generate
device directory when we probe device driver.

And we remove the root directory at hisi_sas_exit(),
but recursively delete device directory when we remove device driver.
Signed-off-by: NLuo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: NJohn Garry <john.garry@huawei.com>