x86/io_delay: Break instead of fallthrough in switch statement

The current code is fine since 'case CONFIG_IO_DELAY_TYPE_NONE'
does nothing, but scripts/checkpatch.pl complains about this:

  warning: Possible switch case/default not preceded by break or fallthrough comment

I like break statement better than a fallthrough comment here.
It avoids the warning and clarify the code.

No behavior change is intended.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20190521072211.21014-1-yamada.masahiro@socionext.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/io_delay.c

@@ -39,6 +39,7 @@ void native_io_delay(void)
 		 * are shorter until calibrated):
 		 */
 		udelay(2);
+		break;
 	case CONFIG_IO_DELAY_TYPE_NONE:
 		break;
 	}