nl802154: add support for cca ed level info

This patch adds information about the current cca ed level when the phy
is dumped over nl802154.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>