Bluetooth: Controller: Fix single timer s/w switch when using DPPI

Fix single timer s/w switch when using DPPI on nRF53/54L
where the previous compare register index is incorrect when
the base index is a odd numbered DPPI used.

The issue is discovered as failing ISO Receiver sample with
Coded PHY and single timer use, where the compare base is
configured to use the DPPI 3 (instead of 4).

Upstream samples do not use single timer for s/w switch, the
issue is discovered when adding support for nRF54L which has
to use single timer due to the radio only having a single
timer.

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