mmc: mmc_test: Fix removal of debugfs file

In __mmc_test_register_dbgfs_file(), we need to assign 'file', as it's
being used when removing the debugfs files when the mmc_test module is
removed.

Fixes: a04c50aa ("mmc: core: no need to check return value of debugfs_create functions")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org
[Ulf: Re-wrote the commit msg]
Link: https://lore.kernel.org/r/20221123095506.1965691-1-yebin@huaweicloud.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>