Bluetooth: controller: Fix RSSI sign
Bluetooth: controller: Fix RSSI sign

The RSSI value is an 8-bit signed integer. Since the Link Layer works
only with positive unsigned integers, translate into a negative number
at the HCI layer.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

aceofall/zephyr-iotos,runchip/zephyr-cc3220,zephyriot/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,punitvara/zephyr,fbsder/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyriot/zephyr,explora26/zephyr,explora26/zephyr,fbsder/zephyr,fbsder/zephyr,ldts/zephyr,nashif/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,kraj/zephyr,mbolivar/zephyr,explora26/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,punitvara/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,runchip/zephyr-cc3220,ldts/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,nashif/zephyr,kraj/zephyr,runchip/zephyr-cc3220,zephyriot/zephyr,kraj/zephyr,aceofall/zephyr-iotos,galak/zephyr,mbolivar/zephyr,nashif/zephyr,Vudentz/zephyr,mbolivar/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,runchip/zephyr-cc3220,fbsder/zephyr