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: qemu-stable@nongnu.org
Buglink: https://bugs.launchpad.net/qemu/+bug/1892960


Fixes: d7dfca08 ("hw/sdhci: introduce standard SD host controller")
Reported-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20200901140411.112150-3-f4bug@amsat.org>