wifi: cfg80211: fix buffer overflow in elem comparison

For vendor elements, the code here assumes that 5 octets
are present without checking. Since the element itself is
already checked to fit, we only need to check the length.

Reported-and-tested-by: Sönke Huster <shuster@seemoo.tu-darmstadt.de>
Fixes: 0b8fb823 ("cfg80211: Parsing of Multiple BSSID information in scanning")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>