only try sector 0 on a floppy.

Reference: /n/atom/patch/applied2013/iplsec0
Date: Thu Oct  3 20:20:59 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/boot/pcipl/fat.c	Thu Oct  3 20:20:49 2013
+++ /sys/src/boot/pcipl/fat.c	Thu Oct  3 20:20:49 2013
@@ -339,7 +339,7 @@
 		return -1;
 	if(buf[0x1fe] != 0x55 || buf[0x1ff] != 0xAA)
 		return -1;
-	if(lba == 0){
+	if(lba == 0 && (drive & 0x80) == 0){	/* floppy */
 		fat->drive = drive;
 		fat->partlba = 0;
 		if(!conffat(fat, buf))
@@ -360,6 +360,7 @@
 		default:
 			if(p[i].status != 0x80)
 				continue;
+		case 0x39:	/* always try plan9 partition */
 			fat->drive = drive;
 			fat->partlba = lba + GETLONG(p[i].lba);
 			if(readsect(drive, fat->partlba, buf))