PCI: Add SPDX GPL-2.0+ to replace implicit GPL v2 or later statement

7441b062 ("s390/pci: PCI hotplug support via SCLP") added
s390_pci_hpc.c, which included this license information:

  +MODULE_LICENSE("GPL");

Based on "git show 7441b062:include/linux/module.h", that "GPL" string
means "GPL v2 or later":

   *      "GPL"                           [GNU Public License v2 or later]

0729dcf2 ("s390: hotplug: make pci_hpc explicitly non-modular")
subsequently replaced the MODULE_LICENSE() with a "License: GPL" comment.

Add SPDX GPL-2.0+ and remove the "License: GPL" comment, relying on the
assertion in b2441318 ("License cleanup: add SPDX GPL-2.0 license
identifier to files with no license") that the SPDX identifier may be used
instead of the full boilerplate text.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Sebastian Ott <sebott@linux.vnet.ibm.com>