sd: sdhci: check data length during dma_memory_read

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: [email protected]
Reported-by: Jiang Xin <[email protected]>
Signed-off-by: Prasad J Pandit <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Message-id: 20170130064736[email protected]
Signed-off-by: Peter Maydell <[email protected]>