firmware-tools/ptgen: fix endianness conversion for PTE .start, .length field.

They are of 32-bit little endian integers.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45956 3c298f89-4303-0410-b956-a3cf2f4a3e73