wifi: rtw89: phy: move bb_gain_info used by WiFi 6 chips to union

WiFi 7 chips use different bb_gain_info struct, so move existing struct to
a union in advance. This doesn't change logic at all.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240105064228.36580-2-pkshih@realtek.com