If PIO mode should be auto-tuned xferspeed argument for svwks_tune_chipset()
is equal to 255 (0xFF).  It is then passed to ide_rate_filter() which matches
desired mode with chipset capabilities.  Since 255 is greater than any of the
values used for transfer modes, ide_rate_filter() will always return the
highest mode supported by both device/chipset (which sometimes should not be
used ie. when host is a simplex one) and the wrong mode will be set.

Thanks to Torben Mathiasen and Tomas Szepe for testing it.