mmc: sh_mmcif: Correct TX DMA channel allocation

Commit 27cbd7e8 ("mmc: sh_mmcif: rework dma channel handling")
introduced a typo causing the TX DMA channel allocation to be overwritten
by the requested RX DMA channel.

Fixes: 27cbd7e8 ("mmc: sh_mmcif: rework dma channel handling")
Signed-off-by: Chris Paterson <chris.paterson2@renesas.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>