x86: Fix ASM_X86__ header guards

Change header guards named "ASM_X86__*" to "_ASM_X86_*" since:

a. the double underscore is ugly and pointless.
b. no leading underscore violates namespace constraints.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/arch/x86/include/asm/serial.h b/arch/x86/include/asm/serial.h
index 303660b..628c801 100644
--- a/arch/x86/include/asm/serial.h
+++ b/arch/x86/include/asm/serial.h
@@ -1,5 +1,5 @@
-#ifndef ASM_X86__SERIAL_H
-#define ASM_X86__SERIAL_H
+#ifndef _ASM_X86_SERIAL_H
+#define _ASM_X86_SERIAL_H
 
 /*
  * This assumes you have a 1.8432 MHz clock for your UART.
@@ -26,4 +26,4 @@
 	{ 0, BASE_BAUD, 0x3E8, 4, STD_COM_FLAGS },	/* ttyS2 */	\
 	{ 0, BASE_BAUD, 0x2E8, 3, STD_COM4_FLAGS },	/* ttyS3 */
 
-#endif /* ASM_X86__SERIAL_H */
+#endif /* _ASM_X86_SERIAL_H */