MIPS: AR7: Override CFLAGS with -Werror

Now that we have removed all warnings from the ar7 board code we can use
-Werror like on other MIPS boards.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>