mac80211: fix skb length check in ieee80211_scan_rx()

In the Linux kernel, the following vulnerability has been resolved:

mac80211: fix skb length check in ieee80211_scan_rx()

Replace hard-coded compile-time constants for header length check with dynamic determination based on the frame type. Otherwise, we hit a validation WARN_ON in cfg80211 later.

[style fixes, reword commit message]