x86: mtrr: cyrix: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: i386_defconfig i386):

arch/x86/kernel/cpu/mtrr/cyrix.c:99:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20190805201712.GA19927@embeddedor

arch/x86/kernel/cpu/mtrr/cyrix.c

@@ -98,6 +98,7 @@ cyrix_get_free_region(unsigned long base, unsigned long size, int replace_reg)
 	case 7:
 		if (size < 0x40)
 			break;
+		/* Else, fall through */
 	case 6:
 	case 5:
 	case 4: