wifi: rtw89: adjust DIG threshold to reduce false alarm

Use RSSI without subtracting offset as packet detection lower bound and
set an absolute minimal threshold. It's equivalent to setting a higher
noise floor, thereby reducing false alarm and improving interference
endurance.

Signed-off-by: Eric Huang <echuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240828055217.10263-2-pkshih@realtek.com