rtw88: coex: add separate flag for manual control

The original stop_dm flag is shared with the wifi and coexistence
mechanism internal usage, which represent two meanings, and makes
developers hard to debug.
Add a new flag manual_control for coexistence mechanism usage only.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201109085909.9143-7-pkshih@realtek.com