staging: rtl8192u: remove null check after call container_of()

container_of() will never return NULL, so remove useless code.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1652696533-18011-1-git-send-email-baihaowen@meizu.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>