PCI: Constify sysfs attribute_group structures

PCI: Constify sysfs 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
   8480	   2024	      4	  10508	   290c	drivers/pci/pci-sysfs.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   8736	   1768	      4	  10508	   290c	drivers/pci/pci-sysfs.o

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