Change the cmd_trigger op to include the write of the command buffer.

Also, instead of letting the lower driver access the cmd_box_addr element
directly, we now pass the address in the trigger_cmd operation, so it
doesn't have to be exported.
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>