staging: rtl8188eu: remove a dead assignment

There's no need to set cmd_hdl to NULL after it's used. It will be set
again before the next command is processed.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210408195601.4762-11-martin@kaiser.cx


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