Bluetooth: Controller: Fix event overhead check for single timer use

Single timer use needs additional event overhead to start
radio to ensure the software switch implementation executing
in radio completion ISR can correctly set the tIFS switching
after a maximum radio ISR latency.

The latency value shall be the used additional overhead so
that there is no spurious tIFS switch before radio ISR
executes.

The additional event overhead value is not considered in the
event overhead check.

Relates to commit 37bf99ee ("Bluetooth: Controller: Fix
radio_tmr_start_us for single timer use").

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