wifi: rtw89: add support for negative values of dBm to linear conversion

Enhanced the dBm to linear conversion function to accommodate negative
dBm values and improved the precision from 1 dBm to 0.25 dBm.

Signed-off-by: Kuan-Chung Chen <damon.chen@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250306021144.12854-2-pkshih@realtek.com