staging: rtl8712: handle firmware load failure

when firmware fails to load we should not call unregister_netdev()
this patch fixes a race condition between rtl871x_load_fw_cb() and
r871xu_dev_remove() and fixes the bug reported by syzbot

Reported-by:  <syzbot+80899a8a8efe8968cde7@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=80899a8a8efe8968cde7


Signed-off-by: Rustam Kovhaev <rkovhaev@gmail.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200716151324.1036204-1-rkovhaev@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>