Bluetooth: controller: Fix master role RSSI measurement

Fix broken master role RSSI measurement. Since the original
contribution clean up into Zephyr, the radio shorts that was
set for measuring the RSSI for master role has been broken,
as it was cleared by the radio switching code further in the
Tx ISR.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>