mmc: bcm2835: stop setting chan_config->slave_id

mmc: bcm2835: stop setting chan_config->slave_id

The field is not interpreted by the DMA engine driver, as all the data
is passed from devicetree instead. Remove the assignment so the field
can eventually be deleted.

Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211122222203.4103644-5-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>