ARM: OMAP: Fix chain_a_transfer return value

This patch changes the return value of omap_dma_chain_a_transfer
to 0 on success instead of the flag 'start_dma', which wasn't really useful
for anything.

Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>