hw/sd.c: On CRC error, set CRC error status bit rather than clearing it

If we fail to validate the CRC for an SD command we should be setting
COM_CRC_ERROR, not clearing it. (This bug actually has no effect currently
because sd_req_crc_validate() always returns success.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
