dmaengine: sprd: stop referencing config->slave_id

It appears that the code that reads the slave_id from the channel config
was copied incorrectly from other drivers. Nothing ever sets this field
on platforms that use this driver, so remove the reference.

Reviewed-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211122222203.4103644-8-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>