Fix bashism in init script (Closes: #530191).

debian/init.d

@@ -36,7 +36,7 @@ if [ -f $DEFAULT ] ; then
 	. $DEFAULT
 fi
 
-if [ "$INTERFACE" == "" ] ; then
+if [ "$INTERFACE" = "" ] ; then
     echo "Not starting $NAME"
     echo "Edit $DEFAULT and define the INTERFACE variable"
     exit 0