wifi: cfg80211: fix ieee80211_data_to_8023_exthdr handling of small packets

STP topology change notification packets only have a payload of 7 bytes,
so they get dropped due to the skb->len < hdrlen + 8 check.
Fix this by removing the extra 8 from the skb->len check and checking the
return code on the skb_copy_bits calls.

Fixes: 2d1c304c

 ("cfg80211: add function for 802.3 conversion with separate output buffer")
Reported-by: Chad Monroe <chad.monroe@smartrg.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>