drivers: ieee802154: cc13xx_cc26xx_subg: Append CRC only for valid packets

During testing, rx_done() callback was seen to be invoked with zero-length packets, when RAW_MODE is enabled, CRC is being appended to these packets thus making downstream consumers think the packet is valid.Root cause of the callback being invoked with zero-length packet is unknown, but appending CRC to these packets is incorrect, thus fix it.

Signed-off-by: Vaishnav Achath vaishnav.a@ti.com