Bluetooth: controller: enable missing RSSI while periodic adv sync

The RSSI measurement was not enabled while receiving periodic
advertising. The function responsible for enable the feature
in radio was called, but it was done too early.
It was overwritten by radio_switch_XXX function that assigns
a value to RADIO->SHORTS register.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>