greybus: sdio: fix tear-down use-after-free

The mmc-driver private data must not be accessed after mmc_free_host()
has released it.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>