staging: r8188eu: calculate the addba response length

An addba response always ends with the timeout field. The length of the
addba response is the offset of the end of the timeout field 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 response components.

Tested-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220602193726.280922-6-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>