rtw89: 8852c: add efuse gain offset parser

Define efuse struct to access gain offset, and store them for further use
by setting channel.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220414062027.62638-8-pkshih@realtek.com