cxl/core/port: Fix NULL but dereferenced coccicheck error

Fix the following coccicheck warning:
./drivers/cxl/core/port.c:913:21-24: ERROR: port is NULL but dereferenced.

The put_device() is only relevent in the is_cxl_root() case.

Fixes: 2703c16c

 ("cxl/core/port: Add switch port enumeration")
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Link: https://lore.kernel.org/r/20220307094158.404882-1-wanjiabing@vivo.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>