mmc: MMC_USDHI6ROL0 should depend on HAS_DMA

If NO_DMA=y:

    drivers/built-in.o: In function `usdhi6_dma_setup':
    usdhi6rol0.c:(.text+0x5c35fc): undefined reference to `dma_map_sg'
    drivers/built-in.o: In function `usdhi6_dma_stop_unmap':
    usdhi6rol0.c:(.text+0x5c3738): undefined reference to `dma_unmap_sg'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>