hw/sd/sdcard: Fix error case for CMD18

In commit 468fa450a7e0 ("hw/sd: Switch read/write primitive to buf+len"), `sd_read_byte()` changed its contract to return the read size rather than the read value (and was renamed to `sd_read_data()` accordingly). In an error case, however, `sd_read_data()` returns 0 by means of `dummy_byte` which is the code for the old contract. Moreover, `sdbus_read_data()` asserts the virtual method `read_data()` (and thus `sd_read_data()`) to return a non-zero size, i.e. to make progress and not loop forever. Fix the code to behave like the "DAT read illegal for command" case. Fixes: 468fa450a7e0 ("hw/sd: Switch read/write primitive to buf+len") Reviewed-by: Bin Meng <bin.meng@processmission.com> Signed-off-by: Bernhard Beschow <shentey@gmail.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com> Message-ID: <20260720201133.24796-2-shentey@gmail.com> Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>