Move command descriptor setup to seperate function as preparation
for the following DMA setup fixes.
    
Note 1: also fix a harmless typo while moving it: sa_idx is initialized
	to dma->resi instead of dma->cmdi.
    
Note 2: errors from command descriptor setup are not propagated back,
	anymore, they can't be handled anyway and all conditions leading
	to errors should be checked earlier.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Acked-by: NEvgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>