Some usb flash drives report silly number of LUNs (e.g. 80) even though
the maximum allowed is 15.  Fix by masking off high order bits in maxlun.

Reference: /n/sources/patch/applied/usb-disk-maxlun
Date: Sat May 22 16:35:13 CES 2010
Signed-off-by: miller@hamnavoe.com

--- /sys/src/cmd/usb/disk/disk.c	Sat May 22 16:33:20 2010
+++ /sys/src/cmd/usb/disk/disk.c	Sat May 22 16:33:17 2010
@@ -59,7 +59,7 @@
 		dprint(2, "disk: %s: getmaxlun failed: %r\n", dev->dir);
 	}else
 		dprint(2, "disk: %s: maxlun %d\n", dev->dir, max);
-	return max;
+	return max&0xF;
 }
 
 static int