mmc: debugfs: Fix file release memory leak

When using single_open() for opening, single_release() should be
used instead of seq_release(), otherwise there is a memory leak.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20220608090152.179395-1-weiyongjun1@huawei.com


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