wifi: rtw89: use struct to access register-based H2C/C2H

The register-based H2C/C2H are used to exchange commands and events with
firmware. The exchange data is limited, but it is relatively simple,
because it can work before HCI initialization. To make these code clean,
use struct to access them. This patch doesn't change logic at all.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230522122513.13559-6-pkshih@realtek.com