MIPS: Lantiq: Fix setting the PCI bus speed on AR9

The bits used to set the PCI bus speed on AR9 are slightly different to
those used on Danube.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2614/


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