mmc: atmel-mci: Add description for struct member

Add description for dma filtering function in struct
mci_platform_data. Fixes the warning: Function parameter or member
'dma_filter' not described in 'mci_platform_data'.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309020937.C5S2sRnr-lkp@intel.com/Signed-off-by: Balamanikandan Gunasundar <balamanikandan.gunasundar@microchip.com>
Link: https://lore.kernel.org/r/20230911092540.76334-1-balamanikandan.gunasundar@microchip.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/atmel-mci.c

@@ -227,6 +227,7 @@ struct mci_slot_pdata {
 /**
  * struct mci_platform_data - board-specific MMC/SDcard configuration
  * @dma_slave: DMA slave interface to use in data transfers.
+ * @dma_filter: Filtering function to filter the DMA channel
  * @slot: Per-slot configuration data.
  */
 struct mci_platform_data {