mmc: mmc_test: avoid stalled file in debugfs

During card removal and inserting cycle the test file in the debugfs could be
stalled until the host driver removes it. Let's keep the file in the linked
list and destroy it when card is removed.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Per Forlin <per.forlin@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>