[HIFN]: Move command descriptor setup to seperate function

[HIFN]: Move command descriptor setup to seperate function

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: Patrick McHardy <kaber@trash.net>
Acked-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>