ramips: gdma: remove slave_id field

ramips: gdma: remove slave_id field

Fix compile error:
drivers/dma/ralink-gdma.c: In function 'gdma_dma_config':
drivers/dma/ralink-gdma.c:197:40: error: 'struct dma_slave_config' has no member named 'slave_id'
  197 |                 chan->slave_id = config->slave_id;
      |                                        ^~
drivers/dma/ralink-gdma.c:206:40: error: 'struct dma_slave_config' has no member named 'slave_id'
  206 |                 chan->slave_id = config->slave_id;
      |                                        ^~
make[8]: *** [scripts/Makefile.build:243: drivers/dma/ralink-gdma.o] Error 1

ref: https://lore.kernel.org/all/20211122222203.4103644-1-arnd@kernel.org/

Signed-off-by: Shiji Yang <yangshiji66@qq.com>