Old code for changing DMA size from the ( possibly smaller ) first cycle
was too expensive in some sense, causing momentary channel hopping on the
ADC when run at 65k and time_shift=0.

The new code just changes the size value of an already existing DMA
module instead of totally recreating it.

type files to plx_9056 file, which now contains all ADC/DAC DMA functions.

common defs and functions in 2 new files: gsc_adc_common.c and
gsc_adc_common.h.

- Moved common ADC DMA functions to new plx_9056.c (Chip that controls DMA)
- Removed redundant DMA functions from ADC drivers.