Make script work correctly when there are more than one usb drive.

Reference: /n/sources/patch/applied/usbfat-multiple
Date: Sat May 22 16:31:47 CES 2010
Signed-off-by: miller@hamnavoe.com

--- /rc/bin/usbfat:	Sat May 22 16:27:50 2010
+++ /rc/bin/usbfat:	Sat May 22 16:27:47 2010
@@ -29,11 +29,11 @@
 }
 
 if (~ $#disks 0){
-	if(! test -e /dev/sdU*/data){
+	disks = /dev/sdU*/data
+	if(! test -e $disks(1)){
 		echo no usb disks >[1=2]
 		exit nodisk
 	}
-	disks = `{echo /dev/sdU*/data}
 }
 for(d in $disks){
 	if(~ $d sdU*.[0-9]*)