Bluetooth: Controller: Use direct ISR for Radio IRQ only

The new hard real-time (direct) interrupt support is used
only for Radio IRQ. Only Radio ISR has hard real-time
requirement of completing its execution inside the tIFS of
150us.

This commit reverts back RTC0 and SWI4 ISR to using the
normal IRQ_CONNECT.

Observed on nRF51, Radio ISR (open text, notifications):
Latencies: min. of 5us and max. of 10us.
CPU utilized: min. of 44us and max. of 77us.

Observed on nRF51, Radio ISR (LESC, fast encrypted,
notifications):
Latencies: min. of 5us and max. of 10us.
CPU utilized: min. of 39us and max. of 112us.

Jira: ZEP-1038

Change-id: Id3d09df7bdbdfea090f21f6f58aaded8c5f1e10d
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>