93cx6 datasheet available here:

        http://ww1.microchip.com/downloads/en/DeviceDoc/21749F.pdf

Figure 1-1 and Table 1-2 on pages 4-5 indicate that both Clock High
Time and Clock Low Time have largest minimum times of 450ns.
Signed-off-by: John W. Linville <linville@tuxdriver.com>