LEDs brightness is now only updated if the value actually changed, thus reducing
the number of fwrite syscalls significantly.

Signed-off-by: Daniel Golle <dgolle@allnet.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@33766 3c298f89-4303-0410-b956-a3cf2f4a3e73