dmaengine: fix incorrect kernel-doc comment for struct dma_slave_config

The 'direction' member of 'struct dma_slave_config' is of data
type 'enum dma_transfer_direction', so update the kernel-doc comment
for 'struct dma_slave_config' to refer to appropriate values.

Signed-off-by: Alexander Popov <a13xp0p0v88@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>