MIPS: constify sysrq_key_op

With earlier commits, the API no longer discards the const-ness of the
sysrq_key_op. As such we can add the notation.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-kernel@vger.kernel.org
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://lore.kernel.org/r/20200513214351.2138580-5-emil.l.velikov@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/mips/kernel/sysrq.c

@@ -52,7 +52,7 @@ static void sysrq_handle_tlbdump(int key)
 #endif
 }
 
-static struct sysrq_key_op sysrq_tlbdump_op = {
+static const struct sysrq_key_op sysrq_tlbdump_op = {
 	.handler        = sysrq_handle_tlbdump,
 	.help_msg       = "show-tlbs(x)",
 	.action_msg     = "Show TLB entries",