Do not check sequence number for QoS Null frames; set it for generated QoS Null
frames to 0

From IEEE Std. 802.11-2012, 8.3.2.1 "Data frame format", p. 415 (513):
"The Sequence Control field for QoS (+)Null frames is ignored by the receiver
upon reception."

At this moment, any <mode>_input() function interprets them as regular QoS data
frames with TID = 0. As a result, stations, that use another TX sequence for
QoS Null frames (e.g. wpi(4), where (QoS) Null frames are generated by the
firmware), may experience significant packet loss with any other NIC in hostap
mode.

Tested:

• wpi(4) (author)
• iwn(4) - Intel 5100, STA mode (me)

PR: kern/200128
Submitted by: Andriy Voskoboinyk <s3erios@gmail.com>