PCI/ASPM: Add missing newline in sysfs 'policy'

When I cat ASPM parameter 'policy' by sysfs, it displays as follows.  Add a
newline for easy reading.  Other sysfs attributes already include a
newline.

  [root@localhost ~]# cat /sys/module/pcie_aspm/parameters/policy
  [default] performance powersave powersupersave [root@localhost ~]#

Fixes: 7d715a6c

 ("PCI: add PCI Express ASPM support")
Link: https://lore.kernel.org/r/1594972765-10404-1-git-send-email-wangxiongfeng2@huawei.com
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>