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.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>