fix silly error in fis, noticed by forsyth/geoff.

Reference: /n/atom/patch/applied/misfis
Date: Sun Sep 13 15:50:10 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libfis/fis.c	Sun Sep 13 15:45:18 2015
+++ /sys/src/libfis/fis.c	Sun Sep 13 15:45:19 2015
@@ -441,7 +441,7 @@
 
 	c[Flba24] = lba >> 24;
 	c[Flba32] = lba >> 32;
-	c[Flba40] = lba >> 48;
+	c[Flba40] = lba >> 40;
 	c[Ffeat8] = 0;
 
 	c[Fsc] = nsect;