wifi: rtw89: correct PHY register offset for PHY-1

wifi: rtw89: correct PHY register offset for PHY-1

PHY-1 can be seen as a copy of PHY-0, and the difference is their base
register address, so add a function to get offset to access PHY-1.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240209065229.34515-2-pkshih@realtek.com