wifi: rtw89: use struct and le32_get_bits() to access RX descriptor

RX descriptor is to provide basic and important information related to
packets, such as packet size, security, MAC ID and so on. Change to use
struct to access these fields, and not 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/20230522122513.13559-5-pkshih@realtek.com