staging: rtl8712: use list_first_entry_or_null()

Use list_first_entry_or_null() instead of list_empty() + LIST_CONTAINOR()
to simplify the code.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>