hw/sd.c: Handle illegal commands in sd_do_command

Add an extra sd_illegal value to the sd_rsp_type_t enum so that
sd_app_command() and sd_normal_command() can tell sd_do_command()
that the command was illegal. This is needed so we can do things
like reset certain status bits only on receipt of a valid command.
For the moment, just use it to pull out the setting of the
ILLEGAL_COMMAND status bit into sd_do_command().

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