rtw88: coex: change the decode method from firmware

Fix sometimes FW information will be parsed as wrong value,
do a correction of sign bit to show the correct information.
(Ex, Value should be 20, but it shows 236.)

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/20201112031430.4846-12-pkshih@realtek.com