PCI/sysfs: Constify struct kobj_type pci_slot_ktype

Since commit ee6d3dd4 ("driver core: make kobj_type constant.") the
driver core allows the usage of const struct kobj_type.

Take advantage of this to constify the structure definition to prevent
modification at runtime.

Link: https://lore.kernel.org/r/20230216-kobj_type-pci-v1-1-46a63c8612b5@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>