investigate 30MHz SPI clock panics for DAC if clk line is loaded by scope probe

[nkrackow]

The HAL would sometime panic with a 30 MHz SPI clock if a scope probe is placed on the clock line.
https://github.com/quartiq/thermostat-eem/blob/master/src/hardware/dac.rs#L24
To be safe the clock was left at 8 MHz for now.