wifi: rtw89: add chip_ops::chan_to_rf18_val to get code of RF register value

The RF 0x18 register stores radio frequency domain parameters, including
band, center channel and bandwidth. This information is used in RF
domain. Add a chip_ops to retrieve the RF 0x18 value, which allows
driver to query for a specific channel.

No logic is changed.

Signed-off-by: Kuan-Chung Chen <damon.chen@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250606020408.17035-1-pkshih@realtek.com