Bluetooth: Controller: Nordic LLL: Use HAL to clear RTC event

Use the HAL event clear function to clear the RTC EVENT
instead of accessing the register directly.

This allows using an updated version of the RTC HW models
which generate level interrupts (as the real HW) and in
which if the EVEN register is not properly cleared,
the interrupts are kept high, resulting in the interrupt
handler reentering immediately after exiting.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>