net/ieee802154: Add Sub-Ghz dedicated function on the radio API

Sub-Ghz bands have different limit of channels. 10, or more than a
thousand is actually possible. Thus the device needs to expose such
limit to the L2 which is unaware of frequency band logic. L2 will
then allow user to select a proper channel.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>