rtw88: mark rtw_fw_hdr __packed

The use of u8 and __le16 in this struct assumes that it's going to be
packed to byte alignment. C doesn't guarantee that, so we should mark
this __packed.

Fixes: cc20a713 ("rtw88: use struct rtw_fw_hdr to access firmware header")
Cc: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>