wifi: rtw89: use struct to parse firmware header

A firmware contains basic header, sections and optional dynamic header.
Define them by a struct, so it will be easier to understand the layout,
and also simply access these elements.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230616060601.28460-1-pkshih@realtek.com