wifi: rtw89: coex: Update function to get BT RSSI and hardware counter

Correct Bluetooth RSSI count method. The 6dB is the gap between hardware
packet sampled value and real RSSI value.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230412012831.10519-4-pkshih@realtek.com