usbmon-setperms: do not grant unnecessary write

Also convert tabs to spaces.

usbmon-setperms

@@ -24,10 +24,17 @@ usbmons=$(lsusb -d046d: | cut -c7 | sed 's,^,/dev/usbmon,' | sort -u)
 
 echo Found devices: $usbmons
 
-if [ -n "$usbmons" ]; then
-	echo "Attempting to change permissions..."
-	sudo chgrp -v $GROUP $usbmons
-	sudo chmod -v g+rw $usbmons
+usbmonsw=
+for dev in $usbmons; do
+    if [ ! -r "$dev" ]; then
+        usbmonsw="$usbmonsw $dev"
+    fi
+done
+
+if [ -n "$usbmonsw" ]; then
+    echo "Attempting to make $usbmonsw readable"
+    sudo chgrp -v $GROUP $usbmonsw
+    sudo chmod -v g+r $usbmonsw
 else
     echo "No devices found"
 fi