staging: rtl8723au: Fix incorrect pointer test introduced in previous commit

Commit da290c54 inverted the check of
the return value when converting to use cfg80211_find_ie().

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>