Bluetooth: Controller: Separate Tx and Rx packet timer status reset

Add interface to have separate Tx and Rx packet timer status
reset, so that packet timer can be setup ahead in time for
starting the Tx or Rx subevent and status reset only clears
the respective PPI/DPPI that was used when Tx or Rx
completes.

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