Bluetooth: Controller: nrf53: Fix back-to-back Tx Rx implementation

Back-to-back Tx Rx implementation was incorrect for nRF53
that uses DPPI. Both current and next DPPI channels where
enabled in the implementation which only worked correctly
to have the right tIFS when current and next PDU length
were same.

Fix ensures that the correct current DPPI is subscribed
to by the radio subscribe.

The implementation has been refactor to be able to use the
current sw_tifs_toggle value in the HAL implementation.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>