x86: fix ghost EDD devices in /sys again

> This is regression but old enough. Apparently I had for whatever reasons
> EDD turned off till recently. This is 2.6.27-rc5 just in case.
>
> In 2006 I fixed ghost devices due to buggy BIOS:
>
> http://marc.info/?l=linux-kernel&m=114087765422490&w=2
>
> Later edd.S has been rewritten in C, and apparently this patch has been
> lost:
>
> {pts/1}% ls /sys/firmware/edd
> int13_dev80/  int13_dev84/  int13_dev88/  int13_dev8c/
> int13_dev81/  int13_dev85/  int13_dev89/  int13_dev8d/
> int13_dev82/  int13_dev86/  int13_dev8a/  int13_dev8e/
> int13_dev83/  int13_dev87/  int13_dev8b/  int13_dev8f/
>
> But I have just a single disk. This is the same system BTW.

Some BIOSes do not always set CF on error before return from int13.
The patch adds additional check for status being zero (AH == 0).

This was fixed for edd.S in
http://marc.info/?l=linux-kernel&m=114087765422490&w=2, but lost
again when edd.S was rewritten in C.
Signed-off-by: NAndrey Borzenkov <arvidjaar@mail.ru>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/boot/edd.c

@@ -32,7 +32,9 @@ static int read_mbr(u8 devno, void *buf)
 		     : "+a" (ax), "+c" (cx), "+d" (dx), "+b" (bx)
 		     : : "esi", "edi", "memory");
 
-	return -(u8)ax;		/* 0 or -1 */
+	/* Some BIOSes do not set carry flag on error but still return
+	 * error in AH. The condition below is expected to catch both */
+	return -!!ax;		/* 0 or -1 */
 }
 
 static u32 read_mbr_sig(u8 devno, struct edd_info *ei, u32 *mbrsig)