hw/sd.c: Don't complain about SDIO commands CMD52/CMD53

The SDIO specification introduces new commands 52 and 53.
Handle as illegal command but do not complain on stderr,
as SDIO-aware OSes (including Linux) may legitimately use
these in their probing for presence of an SDIO card.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
