MIPS: lantiq: Return correct value for fpi clock on ar9

Some configurations of AR9 reported the incorrect speed for the fpi bus.

Signed-off-by: Ben Mulvihill <ben.mulvihill@gmail.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11448/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>