Bluetooth: Controller: Make it possible to support only TX or RX DF

Some controllers may support only TXing or RXing CTE.
As all DF features are guarded by BT_CTLR_DF, we need to ensure
that it is possible to select those when only TX or RX is available.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>