mac80211: add a single-transaction driver op to switch contexts

In some cases, when the driver is already using all the channel
contexts it can handle at once, we have to do an in-place switch
(ie. we cannot afford using an extra context temporarily for the
transaction).  But some drivers may not support switching the channel
context assigned to a vif on the fly (ie. without unassigning and
assigning it) while others may only work if the context is changed on
the fly, without unassigning it first.

To allow these different scenarios, add a new driver operation that
let's the driver decide how to handle an in-place switch.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>