ath11k: Build check size of ath11k_skb_cb

It is rather easy to add more entries to ath11k_skb_cb while forgetting the
size limit of ieee80211_tx_info->driver_data. So just check this during the
build to reduce the change of accidental buffer overflow in the skbuff->cb.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201119154235.263250-3-sven@narfation.org