rtw88: use struct rtw_fw_hdr to access firmware header

This commit doesn't change logic at all, just use struct rtw_fw_hdr to
access fixed part of 64 bytes header. Since remaining part is variable
length data of actual firmware, we don't define them within the struct.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>