Bluetooth: Controller: Fix ISR profiling for single timer use

Fix ISR profiling when using single timer for tIFS radio
switching wherein in the timer is cleared on every radio
end. Hence, the captured timer value is the latency and
does not required the radio end timestamp to be subtracted.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
(cherry picked from commit 8ef919a7)