rtlwifi: 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.

This is detected by Coccinelle semantic patch.

Fixes: 610247f4

 ("rtlwifi: Improve debugging by using debugfs")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>