hw/sd.c: Correct handling of APP_CMD status bit

Fix some bugs in our implementation of the APP_CMD status bit:
 * the response to an ACMD should have APP_CMD set, not cleared
 * if an illegal ACMD is sent then the next command should be
   handled as a normal command

This requires that we split "card is expecting an ACMD" from
the state of the APP_CMD status bit (the latter indicates
both "expecting ACMD" and "that was an ACMD").

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