scsi: zfcp: Move the position of put_device()

Place the put_device() call after device_unregister() in both
zfcp_unit_remove() and zfcp_sysfs_port_remove_store() to make it more
natural. put_device() ought to be the last time we touch the object in both
functions.

Add comments after put_device() to make code clearer.

Link: https://lore.kernel.org/r/0a568c7733ba0f1dde28b0c663b90270d44dd540.1618417667.git.bblock@linux.ibm.com
Suggested-by: Steffen Maier <maier@linux.ibm.com>
Suggested-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>