add "channels" PHY command listing frequencies with more details

add "channels" PHY command listing frequencies with more details

Channels (frequencies) are getting more details that users may want to
know about. E.g. it's important to know which frequencies allow using
40/80/160 MHz channels to setup AP properly.

We list channels in "info" command output but it's already quite big and
it was agreed to introduce new command rather than expand the old one.

This patch adds "channels" command printing what was already available
in the "info" plus details about supported channel widths. It also
removes DFS info from the "info" output.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>