wifi: rtw89: use struct to access firmware C2H event header

Firmware C2H events contain two-word header which can indicate category,
class, function and length of received events. Use struct to access them,
and 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/20230728070252.66525-7-pkshih@realtek.com