net80211: add IEEE80211_IS_QOS_NULL()

This will be useful when fixing up the sequence number generation
and checks, as the rules around how sequence numbers are generated
have been clarified in 802.11-2016 and later. QoS-NULL frames are
explicitly marked as "any sequence number".

But for now, just create a macro and use it in the one place
it's currently being used as a check - ath(4).

• Add IEEE80211_IS_QOS_NULL().
• Change the "will this frame go into the TX block-ack window" check in the ath(4) transmit path. Note this changes the check to be more specific, but both paths already had previous checks to ensure they're QoS data frames.

Locally tested:

• ath(4), AR9380, STA mode w/ AMPDU TX/RX enabled and negotiated

Differential Revision: https://reviews.freebsd.org/D47645