rtw89: coex: correct C2H header length

To resolve C2H handle length mismatch, or it will parse the c2h content
out of array.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20211209083229.10815-2-pkshih@realtek.com