mac80211: fix channel context suspend/reconfig handling

Sujith reported warnings with suspend/resume due to
channel contexts. When I looked into it, I realised
that the code was completely broken as it unassigned
the channel contexts when suspending, which actually
means they are destroyed.

Eliad Peller then pointed out that we also need to
remove the channel contexts from the driver. When I
looked into this, I also noticed that the code isn't
handling the virtual monitor interface correctly (if
it exists.)

Fix this by calling just the driver methods (if they
are implemented) instead of using the channel context
management code. Also add reconfiguration for the
virtual monitor interface.

Reported-by: Sujith Manoharan <sujith@msujith.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>