Bluetooth: controller: ll_sw: Fix wrong cmake code that adds radio_df.c

Fix error introduced by pull request:
https://github.com/zephyrproject-rtos/zephyr/pull/31591



Zephyrs CMake extension function zephyr_library_sources_ifdef()
does not allow to use complex conditions to toggle if
sources should be included in build or not.
It allows to use only single variable as an input for
feature_toggle argument because it is double-expanded
in contition statement in the function.

In case there is a requirement that source file is included
when more complicated condition is true we have to fall-back
to reqular if() statement and zephyr_library_sources().

The fix is required because without it radio_df.c source
is never compiled. If DF functionality is enabled build
will fail because of missing functions implementation.

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