MIPS: ralink: Fix usb issue during frequency scaling

If the USB HCD is running and the cpu is scaled too low, then the USB
stops working. Increase the idle speed of the core to fix this if the
kernel is built with USB support.

The "magic" values are taken from the Ralink SDK Kernel.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11441/


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