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>