drivers: ieee802154: cc13xx_cc26xx_subg: Skip CRC for invalid packets

During extended 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.

Also fix a minor error which causes build failure when
CONFIG_NET_L2_IEEE802154_SUB_GHZ is not defined.

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