drivers: ieee802154: gracefully handle invalid Ack timestamp

The nRF IEEE 802.15.4 driver might report a received Ack frame with
invalid timestamp, if the timestamp could not have been taken. The upper
layers are not prepared to handle such a case as they expect that for a
received frame, the timestamp is always present and valid.

This commit detects this situation and handles it gracefully by
reporting the transmission as failed as if no Ack was received.

Signed-off-by: Jędrzej Ciupis <jedrzej.ciupis@nordicsemi.no>