spi/omap2: disable DMA requests before complete()

No actual errors have been found for completing
before disabling DMA request lines, but it just
looks more semantically correct that on our DMA
callback we quiesce the whole thing before stating
transfer is finished.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>