hw/sd: Correct the maximum size of a Standard Capacity SD Memory Card

Per the SD spec, Standard Capacity SD Memory Card (SDSC) supports
capacity up to and including 2 GiB.

Fixes: 2d7adea4 ("hw/sd: Support SDHC size cards")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Sai Pavan Boddu <sai.pavan.boddu@xilinx.com>
Message-Id: <1598021136-49525-2-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>