powerpc/prom_init: Make "default_colors" const

It's never modified.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/prom_init.c

@@ -2205,7 +2205,7 @@ static void __init prom_check_displays(void)
 	ihandle ih;
 	int i;
 
-	static unsigned char default_colors[] = {
+	static const unsigned char default_colors[] = {
 		0x00, 0x00, 0x00,
 		0x00, 0x00, 0xaa,
 		0x00, 0xaa, 0x00,