cfg80211: use for_each_element() for multi-bssid parsing

Use the new for_each_element() helper here, we cannot use
for_each_subelement() since we have a fixed 1 byte before
the subelements start.

While at it, also fix le16_to_cpup() to be get_unaligned_le16()
since we don't know anything about alignment.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>