wifi: rtw89: 8852c: rfk: refine target channel calculation in _rx_dck_channel_calc()

The channel is not possibly 0, so original code is fine. Still want to
avoid Coverity warning, so ensure -32 offset for the channel number which
is larger than 125 only. Actually, don't change logic at all.

Addresses-Coverity-ID: 1628150 ("Overflowed constant")

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20241209042020.21290-1-pkshih@realtek.com