davinci: Correct return value of edma_alloc_channel api

Currently, edma_alloc_channel api is returning the channel
number without prepending the controller on which the
channel was allocated. So, if a channel is allocated on
2nd controller, calls subsequent to edma_alloc_channel would
never know that channel was allocated on the 2nd controller,
and continue to operate on 1st controller, resulting in edma
failure. This patch fixes this issue.

Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>