drivers: ieee802154: nrf5: prevent negative timestamps

The nrf-802154 driver may be unable to acquire a valid timestamp under
rare conditions. In such case, the nrf_802154_received_timestamp_raw
reports time=NRF_802154_NO_TIMESTAMP.
The shim implementation must not calculate the PHR timestamp when
receiving this value, because doing so results in an assert in
ptp_packet.h due to a negative time value.
When the driver is unable to capture the timestamp, the packet is
assigned zero as its timestamp.

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>