altos: Put SPI in slower mode when talking to companion board

The AVR CPU on the other side just can't go very fast. This reduces
the SPI clock by a factor of 16, just under 200kHz. As the companion
commands are reasonably short, this shouldn't have a huge effect on
overall SPI utilization.

Signed-off-by: Keith Packard <keithp@keithp.com>