sd: sdhci: check data length during dma_memory_read

While doing multi block SDMA transfer in routine
'sdhci_sdma_transfer_multi_blocks', the 's->fifo_buffer' starting
index 'begin' and data length 's->data_count' could end up to be same.
This could lead to an OOB access issue. Correct transfer data length
to avoid it.

Cc: qemu-stable@nongnu.org
Reported-by: Jiang Xin <jiangxin1@huawei.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20170130064736.9236-1-ppandit@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>