usbdux: usbduxsigma: fixed timing issues between ADC and DAC

usbdux: usbduxsigma: fixed timing issues between ADC and DAC

When sync ADC and the DAC was running at the same time the ADC
showed spikes in the signal. This happened when just before the
DRDY from the ADC was triggered a DAC interrupt was dealt with.
ADC and DAC share the same SPI bus and priority is now given the
ADC. The DAC values are now first stored in a buffer and
are only send to the DAC once the ADC has finished
converting all channels (start = 0) so that the SPI bus is
definitely quiet for about 100us.

Signed-off-by: Bernd Porr <mail@berndporr.me.uk>
Signed-off-by: Kyle McMartin <kyle@kernel.org>