common: fix the syntax error

There have the syntax error: operand expected on PC with ubuntu 16.04

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Change-Id: I03a58ccd36ea9eafa60dfeae4a15f30d7a4e84d9

common/mkfirmware.sh

@@ -85,7 +85,7 @@ assert_size() {
 
     IMG_SIZE=$(stat -c "%s" "$IMG")
 
-    if [ $PART_SIZE -lt $(( "$IMG_SIZE" / 1024 )) ]; then
+    if [ $PART_SIZE -lt $(( $IMG_SIZE / 1024 )) ]; then
         fatal "error: $IMG's size exceed parameter.txt's limit!"
     fi
 }