drivers: ieee802154_nrf5: fix target time for indirect transmission

After change in RD into 64-bit time, target time must be express in
absolute 64-bit time. Upper layer e.g. OpenThread still utilizes only
LSB of the RD time therefore the conversion is required.
Make sure that target time is absolute 64-bit target time.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>