staging: r8188eu: remove unnecessary null check

container_of is never null, so this null check is
unnecessary.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Link: https://lore.kernel.org/r/20220824080350.221614-1-chi.minghao@zte.com.cn


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