void config_siimage_chipset_for_pio (ide_drive_t * drive, byte set_speed);
Compute the best pio mode we can for a given device. Also honour the timings for the driver when dealing with mixed devices. Some of this is ugly but its all wrapped up here
The SI680 can also do VDMA - we need to start using that
we use the BIOS channel timings to avoid driving the task files too fast at the disk. We need to compute the master/slave drive PIO mode properly so that we can up the speed on a hotplug system.