drivers: ieee802154: cc13/26xx_subg: fix/document non-standard CSMA/CA

The CC13xx/CC26xx Sub-GHz driver announces a hardware CSMA/CA capability
which it provides only partially. This change documents the gap.

The change also fixes two related issues with the current CCA
implementation:

- The given default ED threshold was above the allowed threshold
  defined in the specification.

- The CCA timeout was not calculated according to the requirements
  defined in the standard.

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