mips: txx9: make txx9_sramc_subsys const

Now that the driver core can properly handle constant struct bus_type,
move the txx9_sramc_subsys variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/txx9/generic/setup.c

@@ -762,7 +762,7 @@ void __init txx9_aclc_init(unsigned long baseaddr, int irq,
 {
 }
 
-static struct bus_type txx9_sramc_subsys = {
+static const struct bus_type txx9_sramc_subsys = {
 	.name = "txx9_sram",
 	.dev_name = "txx9_sram",
 };