wifi: rtw89: coex: Not to set slot duration to zero to avoid firmware issue

If the duration set to zero, Wi-Fi firmware will trigger some unexpected
issue when firmware try to enable timer.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250616090252.51098-9-pkshih@realtek.com