Buffalo WHR-G300N has a LED for power status indication, but it is not
connected to the GPIO and cannot be controlled by the kernel. So,
WHR-G300N uses "ROUTER" LED as the system status LED instead.
This commit changes it to use "DIAG" LED insted of "ROUTER" like
WHR-G301N in ath79 target.
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>