staging: rtl8188eu: memory leak in rtw_free_cmd_obj()

We were fixing checkpatch.pl warnings and accidentally reversed this
condition.

Fixes: 5b29aaaa ("staging: rtl8188eu: removes comparison to null")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>