mmc: renesas_sdhi_internal_dmac: mask DMAC interrupts

I have encountered an interrupt storm during the eMMC chip probing (and
the chip finally didn't get detected).  It turned out that U-Boot left
the SDHI DMA interrupts enabled while the Linux driver didn't use those.
Masking those interrupts in renesas_sdhi_internal_dmac_request_dma() gets
rid of both issues...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Fixes: 2a68ea78 ("mmc: renesas-sdhi: add support for R-Car Gen3 SDHI DMAC")
Cc: stable@vger.kernel.org # v4.14+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>