modules: openthread: fix handling tx done when sleep to tx is supported

After `otPlatRadioSleep` was fixed to properly set radio state to sleep,
radios supporting `IEEE802154_HW_SLEEP_TO_TX` capability would not be
able to handle `PENDING_EVENT_TX_DONE` while in sleep state.

This commit fixes such case.

Signed-off-by: Eduardo Montoya <eduardo.montoya@nordicsemi.no>