drivers: ieee802154: nRF5: remove RX PHR offset workaround

Builds upon the newly introduced nrf_802154_phr_timestamp_get() function
to calculate RX timestamps according to the timestamp definitions
introduced in earlier commits and removes the prior workaround to
calculate the start-of-frame message timestamp point.

Fixes: #59245

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>