drivers: ieee802154: cc13xx_cc26xx_subg: fix oper. state

The driver API now distinguishes between operational UP/DOWN states as
required by Zephyr's network API and receiver on/off states as an
internal driver state for improved standard conformance.

This change closes the gap between the driver API requirements and the
implementation in this respect.

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