Bluetooth: controller: correct conditional compile for central FEX

In the switch statement there are two cases guarded by ifdef's.
These cases are not compiled in for the following Kconfig settings:
BT_PERIPHERAL=n
BT_CTRL_PER_INIT_FEAT_XCHG=n
The following function call then becomes dead code
This is fixed here by adding the proper guards around the whole case
statement

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>