staging: r8188eu: calculate the addba request length

staging: r8188eu: calculate the addba request length

When issue_action_BA compiles an addba request, it does not add any
extensions. start_seq_num is the last field of the addba request.

The length of the request is the offset of the end of start_seq_num in the
struct ieee80211_mgmt that defines the message.

Use offsetofend to calculate this offset and drop the intermediate pktlen
increments as we add addba request components. (We have to keep the
increments for other message types until we use offsetofend for them as
well.)

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220524090029.242584-13-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>