UPSTREAM: 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.

Bug: 254441685
Reported-and-tested-by: Sönke Huster <shuster@seemoo.tu-darmstadt.de>
Fixes: 0b8fb8235be8 ("cfg80211: Parsing of Multiple BSSID information in scanning")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
(cherry picked from commit 9f16b5c82a025cd4c864737409234ddc44fb166a)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: Ic2910eb2d150852fa315c31fd978bd8640fa37da