mtd: sh_flctl: remove slave_id settings for DMAEngine

Current sh_flctl sets dma_slave_config :: slave_id field for DMAEngine,
but it is no longer needed. Let's remove it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>