mac80211: add extended channel switching capability if the driver supports CSA

The Extended Channel Switching capability bit in the extended
capabilities element must be set if the driver supports CSA on
non-beaconing interfaces.

Since this capability needs to be set during driver registration, the
extended_capabiliities global variable needs to be moved to the local
structure so that it can be modified.

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