PCI: Constify label attribute_group structures

attribute_groups are not supposed to change at runtime.  All functions
working with attribute_groups provided by <linux/sysfs.h> work with const
attribute_group.  So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
    930	    320	      0	   1250	    4e2	drivers/pci/pci-label.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1058	    192	      0	   1250	    4ca	drivers/pci/pci-label.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>