rtw88: fix seq_file memory leak

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

This is detected by Coccinelle semantic patch.

Fixes: e3037485 ("rtw88: new Realtek 802.11ac driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>