hw/sd/sdhci: Fix DMA Transfer Block Size field

hw/sd/sdhci: Fix DMA Transfer Block Size field

The 'Transfer Block Size' field is 12-bit wide.

See section '2.2.2. Block Size Register (Offset 004h)' in datasheet.

Two different bug reproducer available:
- https://bugs.launchpad.net/qemu/+bug/1892960
- https://ruhr-uni-bochum.sciebo.de/s/NNWP2GfwzYKeKwE?path=%2Fsdhci_oob_write1

Cc: [email protected]
Buglink: https://bugs.launchpad.net/qemu/+bug/1892960
Fixes: d7dfca0807a ("hw/sdhci: introduce standard SD host controller")
Reported-by: Alexander Bulekov <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Prasad J Pandit <[email protected]>
Tested-by: Alexander Bulekov <[email protected]>
Message-Id: <20200901140411[email protected]>