staging: r8188eu: remove NULL check for usb_kill_urb

usb_kill_urb handles a NULL parameter. There's no need for NULL checks
before we call usb_kill_urb.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20230111100201.251905-1-martin@kaiser.cx


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