mac80211_hwsim: allow using channel contexts

To use mac80211_hwsim for testing channel contexts it
has to support them, and for that it has to support
hw scan and hw-remain-on-channel.

Since it's pure software, the off-channel activities
are really not off-channel but listening and sending
on a second channel. Also, the multi-channel isn't
really doing TDM, it's just on both channels at the
same time.

For testing purposes, you can specify the number of
concurrent channels with a module parameter, it is
set to one by default. When set to two or more, the
userspace API for wmediumd is disabled as it has no
provisions for multi-channel yet.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>