cpufreq: Temporarily ignore io_is_busy=1

To speed testing of the new sdhost driver that adapts to changes in
core_freq, hack the on-demand governor to treat io_is_busy=1 as
io_is_busy=0. The io_is_busy feature can still be forced using
io_is_busy=2.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>