hw/sd: sdhci: Limit block size only when SDHC_BLKSIZE register is writable

hw/sd: sdhci: Limit block size only when SDHC_BLKSIZE register is writable

Git-commit: 5cd7aa3451b76bb19c0f6adc2b931f091e5d7fcd
References: bsc#1175144, CVE-2020-17380, bsc#1176681, CVE-2020-25085
References: bsc#1182282, CVE-2021-3409

The codes to limit the maximum block size is only necessary when
SDHC_BLKSIZE register is writable.

Tested-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20210303122639.20004-5-bmeng.cn@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Bruce Rogers <brogers@suse.com>