dmaengine: shdma: remove legacy slave_id parsing

The slave device is picked through either devicetree or a filter
function, and any remaining out-of-tree drivers would have warned
about this usage since 2015.

Stop interpreting the field finally so it can be removed from
the interface.

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/20211122222203.4103644-6-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>