mmc: uniphier-sd: avoid using broken DMA RX channel

host->chan_rx is NULL when UNIPHIER_SD_CAP_BROKEN_DMA_RX quirk flag
is set. In this case, it should not set up DMA.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>