wifi: rtw89: coex: Update Bluetooth slot length when Wi-Fi is scanning

If Wi-Fi isn't connected to AP, the firmware scan will not switch to
operation channel. Bluetooth may not have enough time to TX data. This
patch extend the slot to let Bluetooth can TX more to avoid audio lag.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250616090252.51098-10-pkshih@realtek.com