PCI: acpiphp_ibm: Make ibm_apci_table_attr __ro_after_init

ibm_apci_table_attr is not modified after being initialized by
ibm_acpiphp_init().  It is passed as an argument to the functions
sysfs_{remove/create}_bin_file(), but both the arguments are const.
Add __ro_after_init to its declaration.

[bhelgaas: changelog]
Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>

drivers/pci/hotplug/acpiphp_ibm.c

@@ -107,7 +107,7 @@ static void __exit ibm_acpiphp_exit(void);
 
 static acpi_handle ibm_acpi_handle;
 static struct notification ibm_note;
-static struct bin_attribute ibm_apci_table_attr = {
+static struct bin_attribute ibm_apci_table_attr __ro_after_init = {
 	    .attr = {
 		    .name = "apci_table",
 		    .mode = S_IRUGO,