Bluetooth: Controller: Revert EVENT_OVERHEAD_END_US to original value

Revert back the EVENT_OVERHEAD_END_US value to original
value in initial commit 5c3709c1 ("bluetooth:
controller: Add def's re. update to slot reservation calc").

40 us is a sufficient margin considering ~30 us being the
worst case Rx chain delay for S8 Coded PHY on nRF5x SoCs.

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