dmaengine: remove slave_id config field

All references to the slave_id field have been removed, so remove the
field as well to prevent new references from creeping in again.

Originally this allowed slave DMA drivers to configure which device
is accessed with the dmaengine_slave_config() call, but this was
inconsistent, as the same information is also passed while requesting
a channel, and never changes in practice.

In modern kernels, the device is always selected when requesting
the channel, so the .slave_id field is no longer useful.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211122222203.4103644-12-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>