net: openthread: Fix exiting diag when transmission is ongoing

Crash was observed if "diag stop" was invoked during the transmission.
The issue was caused by an update to otPlatRadioSleep that made it
compilant with the documentation and not allowing direct Transmit to
Sleep transition.
The transmission result was forwarded to OpenThread making it crash due
to an invalid state.

Signed-off-by: Marek Porwisz <marek.porwisz@nordicsemi.no>