staging: rtlwifi: use single_open and single_release properly

single_open() returns -ENOMEM when malloc failed, so the caller function
rtl_debugfs_open_rw() should not always return 0. In addition, when using
single_open(), we should use single_release() instead of seq_release() in
the file_operations structure to avoid a memory leak.

Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>