MIPS: lantiq: Fix attributes of of_device_id structure

According to the checkpatch the driver structure of_device_id requires
to be const and with attribute __initconst. Change it accordingly.

Signed-off-by: Petr Cvek <petrcvekcz@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: hauke@hauke-m.de
Cc: john@phrozen.org
Cc: linux-mips@vger.kernel.org
Cc: openwrt-devel@lists.openwrt.org
Cc: pakahmar@hotmail.com