wifi: rtw89: coex: Assign value over than 0 to avoid firmware timer hang

If the slot duration is 0, the firmware timer will trigger timer hang at
the timer initializing state in a low rate due to hardware algorithm.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250205013233.10945-2-pkshih@realtek.com